ll
	 */
	private function retrieve_category() {
		$replacement = null;

		if ( ! empty( $this->args->ID ) ) {
			$cat = $this->get_terms( $this->args->ID, 'category' );
			if ( $cat !== '' ) {
				return $cat;
			}
		}

		if ( isset( $this->args->cat_name ) && ! empty( $this->args->cat_name ) ) {
			$replacement = $this->args->cat_name;
		}

		return $replacement;
	}

	/**
	 * Retrieve the category description for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_category_description() {
		return $this->retrieve_term_description();
	}

	/**
	 * Retrieve the date of the post/page/cpt for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_date() {
		$replacement = null;

		if ( $this->args->post_date !== '' ) {
			// Returns a string.
			$replacement = YoastSEO()->helpers->date->format_translated( $this->args->post_date, get_option( 'date_format' ) );
		}
		else {
			if ( get_query_var( 'day' ) && get_query_var( 'day' ) !== '' ) {
				// Returns a string.
				$replacement = get_the_date();
			}
			else {
				if ( single_month_title( ' ', false ) && single_month_title( ' ', false ) !== '' ) {
					// Returns a string.
					$replacement = single_month_title( ' ', false );
				}
				elseif ( get_query_var( 'year' ) !== '' ) {
					// Returns an integer, let's cast to string.
					$replacement = (string) get_query_var( 'year' );
				}
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt excerpt for use as replacement string.
	 * The excerpt will be auto-generated if it does not exist.
	 *
	 * @return string|null
	 */
	private function retrieve_excerpt() {
		$replacement = null;
		$locale      = \get_locale();

		// Japanese doesn't have a jp_JP variant in WP.
		$limit = ( $locale === 'ja' ) ? 80 : 156;

		// The check `post_password_required` is because excerpt must be hidden for a post with a password.
		if ( ! empty( $this->args->ID ) && ! post_password_required( $this->args->ID ) ) {
			if ( $this->args->post_excerpt !== '' ) {
				$replacement = wp_strip_all_tags( $this->args->post_excerpt );
			}
			elseif ( $this->args->post_content !== '' ) {
				$content = strip_shortcodes( $this->args->post_content );
				$content = wp_strip_all_tags( $content );

				if ( mb_strlen( $content ) <= $limit ) {
					return $content;
				}

				$replacement = wp_html_excerpt( $content, $limit );

				// Check if the description has space and trim the auto-generated string to a word boundary.
				if ( strrpos( $replacement, ' ' ) ) {
					$replacement = substr( $replacement, 0, strrpos( $replacement, ' ' ) );
				}
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt excerpt for use as replacement string (without auto-generation).
	 *
	 * @return string|null
	 */
	private function retrieve_excerpt_only() {
		$replacement = null;

		// The check `post_password_required` is because excerpt must be hidden for a post with a password.
		if ( ! empty( $this->args->ID ) && $this->args->post_excerpt !== '' && ! post_password_required( $this->args->ID ) ) {
			$replacement = wp_strip_all_tags( $this->args->post_excerpt );
		}

		return $replacement;
	}

	/**
	 * Retrieve the title of the parent page of the current page/cpt for use as replacement string.
	 * Only applicable for hierarchical post types.
	 *
	 * @todo Check: shouldn't this use $this->args as well ?
	 *
	 * @return string|null
	 */
	private function retrieve_parent_title() {
		$replacement = null;

		if ( ! empty( $this->args->ID ) ) {
			$parent_id = wp_get_post_parent_id( $this->args->ID );
			if ( $parent_id ) {
				$replacement = get_the_title( $parent_id );
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve the current search phrase for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_searchphrase() {
		$replacement = null;

		$search = get_query_var( 's' );
		if ( $search !== '' ) {
			$replacement = esc_html( $search );
		}

		return $replacement;
	}

	/**
	 * Retrieve the separator for use as replacement string.
	 *
	 * @return string Retrieves the title separator.
	 */
	private function retrieve_sep() {
		return YoastSEO()->helpers->options->get_title_separator();
	}

	/**
	 * Retrieve the site's tag line / description for use as replacement string.
	 *
	 * The `$replacement` variable is static because it doesn't change depending
	 * on the context. See https://github.com/Yoast/wordpress-seo/pull/1172#issuecomment-46019482.
	 *
	 * @return string|null
	 */
	private function retrieve_sitedesc() {
		static $replacement;

		if ( ! isset( $replacement ) ) {
			$description = wp_strip_all_tags( get_bloginfo( 'description' ) );
			if ( $description !== '' ) {
				$replacement = $description;
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve the site's name for use as replacement string.
	 *
	 * The `$replacement` variable is static because it doesn't change depending
	 * on the context. See https://github.com/Yoast/wordpress-seo/pull/1172#issuecomment-46019482.
	 *
	 * @return string|null
	 */
	private function retrieve_sitename() {
		static $replacement;

		if ( ! isset( $replacement ) ) {
			$sitename = YoastSEO()->helpers->site->get_site_name();
			if ( $sitename !== '' ) {
				$replacement = $sitename;
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve the current tag/tags for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_tag() {
		$replacement = null;

		if ( ! empty( $this->args->ID ) ) {
			$tags = $this->get_terms( $this->args->ID, 'post_tag' );
			if ( $tags !== '' ) {
				$replacement = $tags;
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve the tag description for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_tag_description() {
		return $this->retrieve_term_description();
	}

	/**
	 * Retrieve the term description for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_term_description() {
		$replacement = null;

		if ( ! empty( $this->args->term_id ) && ! empty( $this->args->taxonomy ) ) {
			$term_desc = get_term_field( 'description', $this->args->term_id, $this->args->taxonomy );
			if ( $term_desc !== '' ) {
				$replacement = wp_strip_all_tags( $term_desc );
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve the term name for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_term_title() {
		$replacement = null;

		if ( ! empty( $this->args->taxonomy ) && ! empty( $this->args->name ) ) {
			$replacement = $this->args->name;
		}

		return $replacement;
	}

	/**
	 * Retrieve the title of the post/page/cpt for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_title() {
		$replacement = null;

		if ( is_string( $this->args->post_title ) && $this->args->post_title !== '' ) {
			$replacement = $this->args->post_title;
		}

		return $replacement;
	}

	/**
	 * Retrieve primary category for use as replacement string.
	 *
	 * @return bool|int|null
	 */
	private function retrieve_primary_category() {
		$primary_category = null;

		if ( ! empty( $this->args->ID ) ) {
			$wpseo_primary_category = new WPSEO_Primary_Term( 'category', $this->args->ID );

			$term_id = $wpseo_primary_category->get_primary_term();
			$term    = get_term( $term_id );

			if ( ! is_wp_error( $term ) && ! empty( $term ) ) {
				$primary_category = $term->name;
			}
		}

		return $primary_category;
	}

	/**
	 * Retrieve the string generated by get_the_archive_title().
	 *
	 * @return string|null
	 */
	private function retrieve_archive_title() {
		return get_the_archive_title();
	}

	/* *********************** ADVANCED VARIABLES ************************** */

	/**
	 * Determine the page numbering of the current post/page/cpt.
	 *
	 * @param string $request Either 'nr'|'max' - whether to return the page number or the max number of pages.
	 *
	 * @return int|null
	 */
	private function determine_pagenumbering( $request = 'nr' ) {
		global $wp_query, $post;
		$max_num_pages = null;
		$page_number   = null;

		$max_num_pages = 1;

		if ( ! is_singular() ) {
			$page_number = get_query_var( 'paged' );
			if ( $page_number === 0 || $page_number === '' ) {
				$page_number = 1;
			}

			if ( ! empty( $wp_query->max_num_pages ) ) {
				$max_num_pages = $wp_query->max_num_pages;
			}
		}
		else {
			$page_number = get_query_var( 'page' );
			if ( $page_number === 0 || $page_number === '' ) {
				$page_number = 1;
			}

			if ( isset( $post->post_content ) ) {
				$max_num_pages = ( substr_count( $post->post_content, '<!--nextpage-->' ) + 1 );
			}
		}

		$return = null;

		switch ( $request ) {
			case 'nr':
				$return = $page_number;
				break;
			case 'max':
				$return = $max_num_pages;
				break;
		}

		return $return;
	}

	/**
	 * Determine the post type names for the current post/page/cpt.
	 *
	 * @param string $request Either 'single'|'plural' - whether to return the single or plural form.
	 *
	 * @return string|null
	 */
	private function determine_pt_names( $request = 'single' ) {
		global $wp_query;
		$pt_single = null;
		$pt_plural = null;
		$post_type = '';

		if ( isset( $wp_query->query_vars['post_type'] ) && ( ( is_string( $wp_query->query_vars['post_type'] ) && $wp_query->query_vars['post_type'] !== '' ) || ( is_array( $wp_query->query_vars['post_type'] ) && $wp_query->query_vars['post_type'] !== [] ) ) ) {
			$post_type = $wp_query->query_vars['post_type'];
		}
		elseif ( isset( $this->args->post_type ) && ( is_string( $this->args->post_type ) && $this->args->post_type !== '' ) ) {
			$post_type = $this->args->post_type;
		}
		else {
			// Make it work in preview mode.
			$post = $wp_query->get_queried_object();
			if ( $post instanceof WP_Post ) {
				$post_type = $post->post_type;
			}
		}

		if ( is_array( $post_type ) ) {
			$post_type = reset( $post_type );
		}

		if ( $post_type !== '' ) {
			$pt        = get_post_type_object( $post_type );
			$pt_single = $pt->name;
			$pt_plural = $pt->name;
			if ( isset( $pt->labels->singular_name ) ) {
				$pt_single = $pt->labels->singular_name;
			}
			if ( isset( $pt->labels->name ) ) {
				$pt_plural = $pt->labels->name;
			}
		}

		$return = null;

		switch ( $request ) {
			case 'single':
				$return = $pt_single;
				break;
			case 'plural':
				$return = $pt_plural;
				break;
		}

		return $return;
	}

	/**
	 * Retrieve the attachment caption for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_caption() {
		return $this->retrieve_excerpt_only();
	}

	/**
	 * Retrieve a post/page/cpt's custom field value for use as replacement string.
	 *
	 * @param string $var_to_replace The complete variable to replace which includes the name of
	 *                               the custom field which value is to be retrieved.
	 *
	 * @return string|null
	 */
	private function retrieve_cf_custom_field_name( $var_to_replace ) {
		$replacement = null;

		if ( is_string( $var_to_replace ) && $var_to_replace !== '' ) {
			$field = substr( $var_to_replace, 3 );
			if ( ! empty( $this->args->ID ) ) {
				// Post meta can be arrays and in this case we need to exclude them.
				$name = get_post_meta( $this->args->ID, $field, true );
				if ( $name !== '' && ! is_array( $name ) ) {
					$replacement = $name;
				}
			}
			elseif ( ! empty( $this->args->term_id ) ) {
				$name = get_term_meta( $this->args->term_id, $field, true );
				if ( $name !== '' ) {
					$replacement = $name;
				}
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve a post/page/cpt's custom taxonomies for use as replacement string.
	 *
	 * @param string $var_to_replace The complete variable to replace which includes the name of
	 *                               the custom taxonomy which value(s) is to be retrieved.
	 * @param bool   $single         Whether to retrieve only the first or all values for the taxonomy.
	 *
	 * @return string|null
	 */
	private function retrieve_ct_custom_tax_name( $var_to_replace, $single = false ) {
		$replacement = null;

		if ( ( is_string( $var_to_replace ) && $var_to_replace !== '' ) && ! empty( $this->args->ID ) ) {
			$tax  = substr( $var_to_replace, 3 );
			$name = $this->get_terms( $this->args->ID, $tax, $single );
			if ( $name !== '' ) {
				$replacement = $name;
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve a post/page/cpt's custom taxonomies description for use as replacement string.
	 *
	 * @param string $var_to_replace The complete variable to replace which includes the name of
	 *                               the custom taxonomy which description is to be retrieved.
	 *
	 * @return string|null
	 */
	private function retrieve_ct_desc_custom_tax_name( $var_to_replace ) {
		$replacement = null;

		if ( is_string( $var_to_replace ) && $var_to_replace !== '' ) {
			$tax = substr( $var_to_replace, 8 );
			if ( ! empty( $this->args->ID ) ) {
				$terms = get_the_terms( $this->args->ID, $tax );
				if ( is_array( $terms ) && $terms !== [] ) {
					$term      = current( $terms );
					$term_desc = get_term_field( 'description', $term->term_id, $tax );
					if ( $term_desc !== '' ) {
						$replacement = wp_strip_all_tags( $term_desc );
					}
				}
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve the current date for use as replacement string.
	 *
	 * The `$replacement` variable is static because it doesn't change depending
	 * on the context. See https://github.com/Yoast/wordpress-seo/pull/1172#issuecomment-46019482.
	 *
	 * @return string The formatted current date.
	 */
	private function retrieve_currentdate() {
		static $replacement;

		if ( ! isset( $replacement ) ) {
			$replacement = date_i18n( get_option( 'date_format' ) );
		}

		return $replacement;
	}

	/**
	 * Retrieve the current day for use as replacement string.
	 *
	 * The `$replacement` variable is static because it doesn't change depending
	 * on the context. See https://github.com/Yoast/wordpress-seo/pull/1172#issuecomment-46019482.
	 *
	 * @return string The current day.
	 */
	private function retrieve_currentday() {
		static $replacement;

		if ( ! isset( $replacement ) ) {
			$replacement = date_i18n( 'j' );
		}

		return $replacement;
	}

	/**
	 * Retrieve the current month for use as replacement string.
	 *
	 * The `$replacement` variable is static because it doesn't change depending
	 * on the context. See https://github.com/Yoast/wordpress-seo/pull/1172#issuecomment-46019482.
	 *
	 * @return string The current month.
	 */
	private function retrieve_currentmonth() {
		static $replacement;

		if ( ! isset( $replacement ) ) {
			$replacement = date_i18n( 'F' );
		}

		return $replacement;
	}

	/**
	 * Retrieve the current time for use as replacement string.
	 *
	 * The `$replacement` variable is static because it doesn't change depending
	 * on the context. See https://github.com/Yoast/wordpress-seo/pull/1172#issuecomment-46019482.
	 *
	 * @return string The formatted current time.
	 */
	private function retrieve_currenttime() {
		static $replacement;

		if ( ! isset( $replacement ) ) {
			$replacement = date_i18n( get_option( 'time_format' ) );
		}

		return $replacement;
	}

	/**
	 * Retrieve the current year for use as replacement string.
	 *
	 * The `$replacement` variable is static because it doesn't change depending
	 * on the context. See https://github.com/Yoast/wordpress-seo/pull/1172#issuecomment-46019482.
	 *
	 * @return string The current year.
	 */
	private function retrieve_currentyear() {
		static $replacement;

		if ( ! isset( $replacement ) ) {
			$replacement = date_i18n( 'Y' );
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt's focus keyword for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_focuskw() {
		// Retrieve focuskw from a Post.
		if ( ! empty( $this->args->ID ) ) {
			$focus_kw = WPSEO_Meta::get_value( 'focuskw', $this->args->ID );
			if ( $focus_kw !== '' ) {
				return $focus_kw;
			}

			return null;
		}

		// Retrieve focuskw from a Term.
		if ( ! empty( $this->args->term_id ) ) {
			$focus_kw = WPSEO_Taxonomy_Meta::get_term_meta( $this->args->term_id, $this->args->taxonomy, 'focuskw' );
			if ( $focus_kw !== '' ) {
				return $focus_kw;
			}
		}

		return null;
	}

	/**
	 * Retrieve the post/page/cpt ID for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_id() {
		$replacement = null;

		if ( ! empty( $this->args->ID ) ) {
			// The post/page/cpt ID is an integer, let's cast to string.
			$replacement = (string) $this->args->ID;
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt modified time for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_modified() {
		$replacement = null;

		if ( ! empty( $this->args->post_modified ) ) {
			$replacement = YoastSEO()->helpers->date->format_translated( $this->args->post_modified, get_option( 'date_format' ) );
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt author's "nice name" for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_name() {
		$replacement = null;

		$user_id = (int) $this->retrieve_userid();
		$name    = get_the_author_meta( 'display_name', $user_id );
		if ( $name !== '' ) {
			$replacement = $name;
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt author's users description for use as a replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_user_description() {
		$replacement = null;

		$user_id     = (int) $this->retrieve_userid();
		$description = get_the_author_meta( 'description', $user_id );
		if ( $description !== '' ) {
			$replacement = $description;
		}

		return $replacement;
	}

	/**
	 * Retrieve the current page number with context (i.e. 'page 2 of 4') for use as replacement string.
	 *
	 * @return string
	 */
	private function retrieve_page() {
		$replacement = null;

		$max = $this->determine_pagenumbering( 'max' );
		$nr  = $this->determine_pagenumbering( 'nr' );
		$sep = $this->retrieve_sep();

		if ( $max > 1 && $nr > 1 ) {
			/* translators: 1: current page number, 2: total number of pages. */
			$replacement = sprintf( $sep . ' ' . __( 'Page %1$d of %2$d', 'wordpress-seo' ), $nr, $max );
		}

		return $replacement;
	}

	/**
	 * Retrieve the current page number for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_pagenumber() {
		$replacement = null;

		$nr = $this->determine_pagenumbering( 'nr' );
		if ( isset( $nr ) && $nr > 0 ) {
			$replacement = (string) $nr;
		}

		return $replacement;
	}

	/**
	 * Retrieve the current page total for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_pagetotal() {
		$replacement = null;

		$max = $this->determine_pagenumbering( 'max' );
		if ( isset( $max ) && $max > 0 ) {
			$replacement = (string) $max;
		}

		return $replacement;
	}

	/**
	 * Retrieve the post type plural label for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_pt_plural() {
		$replacement = null;

		$name = $this->determine_pt_names( 'plural' );
		if ( isset( $name ) && $name !== '' ) {
			$replacement = $name;
		}

		return $replacement;
	}

	/**
	 * Retrieve the post type single label for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_pt_single() {
		$replacement = null;

		$name = $this->determine_pt_names( 'single' );
		if ( isset( $name ) && $name !== '' ) {
			$replacement = $name;
		}

		return $replacement;
	}

	/**
	 * Retrieve the slug which caused the 404 for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_term404() {
		$replacement = null;

		if ( $this->args->term404 !== '' ) {
			$replacement = sanitize_text_field( str_replace( '-', ' ', $this->args->term404 ) );
		}
		else {
			$error_request = get_query_var( 'pagename' );
			if ( $error_request !== '' ) {
				$replacement = sanitize_text_field( str_replace( '-', ' ', $error_request ) );
			}
			else {
				$error_request = get_query_var( 'name' );
				if ( $error_request !== '' ) {
					$replacement = sanitize_text_field( str_replace( '-', ' ', $error_request ) );
				}
			}
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt author's user id for use as replacement string.
	 *
	 * @return string
	 */
	private function retrieve_userid() {
		// The user ID is an integer, let's cast to string.
		$replacement = ! empty( $this->args->post_author ) ? (string) $this->args->post_author : (string) get_query_var( 'author' );

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt's published year for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_post_year() {
		if ( empty( $this->args->ID ) ) {
			return null;
		}

		return \get_the_date( 'Y', $this->args->ID );
	}

	/**
	 * Retrieve the post/page/cpt's published month for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_post_month() {
		if ( empty( $this->args->ID ) ) {
			return null;
		}

		return \get_the_date( 'F', $this->args->ID );
	}

	/**
	 * Retrieve the post/page/cpt's published day for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_post_day() {
		if ( empty( $this->args->ID ) ) {
			return null;
		}

		return \get_the_date( 'd', $this->args->ID );
	}

	/**
	 * Retrieve the post/page/cpt author's first name for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_author_first_name() {
		$replacement = null;

		$user_id = (int) $this->retrieve_userid();
		$name    = get_the_author_meta( 'first_name', $user_id );
		if ( $name !== '' ) {
			$replacement = $name;
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt author's last name for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_author_last_name() {
		$replacement = null;

		$user_id = (int) $this->retrieve_userid();
		$name    = get_the_author_meta( 'last_name', $user_id );
		if ( $name !== '' ) {
			$replacement = $name;
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt permalink for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_permalink() {
		if ( empty( $this->args->ID ) ) {
			return null;
		}

		return \get_permalink( $this->args->ID );
	}

	/**
	 * Retrieve the post/page/cpt content for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_post_content() {
		$replacement = null;

		// The check `post_password_required` is because content must be hidden for a post with a password.
		if ( ! empty( $this->args->ID ) && $this->args->post_content !== '' && ! post_password_required( $this->args->ID ) ) {
			$content     = strip_shortcodes( $this->args->post_content );
			$replacement = wp_strip_all_tags( $content );
		}

		return $replacement;
	}

	/**
	 * Retrieve the current or first category title. To be used for import data from AIOSEO.
	 * The code derives from AIOSEO's way of dealing with that var, so we can ensure 100% seamless transition.
	 *
	 * @return string|null
	 */
	private function retrieve_category_title() {
		if ( empty( $this->args ) || empty( $this->args->ID ) ) {
			return null;
		}
		$post_id = $this->args->ID;

		$post       = get_post( $post_id );
		$taxonomies = get_object_taxonomies( $post, 'objects' );

		foreach ( $taxonomies as $taxonomy_slug => $taxonomy ) {
			if ( ! $taxonomy->hierarchical ) {
				continue;
			}
			$post_terms = get_the_terms( $post_id, $taxonomy_slug );
			if ( is_array( $post_terms ) && count( $post_terms ) > 0 ) {
				// AiOSEO takes the name of whatever the first hierarchical taxonomy is.
				$term = $post_terms[0];
				if ( $term ) {
					return $term->name;
				}
			}
		}

		return null;
	}

	/* *********************** HELP TEXT RELATED ************************** */

	/**
	 * Set the help text for a user/plugin/theme defined extra variable.
	 *
	 * @param string                     $type                 Type of variable: 'basic' or 'advanced'.
	 * @param WPSEO_Replacement_Variable $replacement_variable The replacement variable to register.
	 */
	private static function register_help_text( $type, WPSEO_Replacement_Variable $replacement_variable ) {
		$identifier = $replacement_variable->get_variable();

		if ( ( is_string( $type ) && in_array( $type, [ 'basic', 'advanced' ], true ) )
			&& ( $identifier !== '' && ! isset( self::$help_texts[ $type ][ $identifier ] ) )
		) {
			self::$help_texts[ $type ][ $identifier ] = $replacement_variable;
		}
	}

	/**
	 * Generates a list of replacement variables based on the help texts.
	 *
	 * @return array List of replace vars.
	 */
	public function get_replacement_variables_with_labels() {
		self::setup_statics_once();

		$custom_variables = [];
		foreach ( array_merge( WPSEO_Custom_Fields::get_custom_fields(), WPSEO_Custom_Taxonomies::get_custom_taxonomies() ) as $custom_variable ) {
			$custom_variables[ $custom_variable ] = new WPSEO_Replacement_Variable( $custom_variable, $this->get_label( $custom_variable ), '' );
		}

		$replacement_variables = array_filter(
			array_merge( self::$help_texts['basic'], self::$help_texts['advanced'] ),
			[ $this, 'is_not_prefixed' ],
			ARRAY_FILTER_USE_KEY
		);

		$hidden = $this->get_hidden_replace_vars();

		return array_values(
			array_map(
				static function ( WPSEO_Replacement_Variable $replacement_variable ) use ( $hidden ) {
					$name = $replacement_variable->get_variable();

					return [
						'name'   => $name,
						'value'  => '',
						'label'  => $replacement_variable->get_label(),
						'hidden' => in_array( $name, $hidden, true ),
					];
				},
				array_merge( $replacement_variables, $custom_variables )
			)
		);
	}

	/**
	 * Generates a list of replacement variables based on the help texts.
	 *
	 * @return array List of replace vars.
	 */
	public function get_replacement_variables_list() {
		self::setup_statics_once();

		$replacement_variables = array_merge(
			$this->get_replacement_variables(),
			WPSEO_Custom_Fields::get_custom_fields(),
			WPSEO_Custom_Taxonomies::get_custom_taxonomies()
		);

		return array_map( [ $this, 'format_replacement_variable' ], $replacement_variables );
	}

	/**
	 * Creates a merged associative array of both the basic and advanced help texts.
	 *
	 * @return array Array with the replacement variables.
	 */
	private function get_replacement_variables() {
		$help_texts = array_merge( self::$help_texts['basic'], self::$help_texts['advanced'] );

		return array_filter( array_keys( $help_texts ), [ $this, 'is_not_prefixed' ] );
	}

	/**
	 * Checks whether the replacement variable contains a `ct_` or `cf_` prefix, because they follow different logic.
	 *
	 * @param string $replacement_variable The replacement variable.
	 *
	 * @return bool True when the replacement variable is not prefixed.
	 */
	private function is_not_prefixed( $replacement_variable ) {
		$prefixes = [ 'cf_', 'ct_' ];
		$prefix   = $this->get_prefix( $replacement_variable );

		return ! in_array( $prefix, $prefixes, true );
	}

	/**
	 * Strip the prefix from a replacement variable name.
	 *
	 * @param string $replacement_variable The replacement variable.
	 *
	 * @return string The replacement variable name without the prefix.
	 */
	private function strip_prefix( $replacement_variable ) {
		return substr( $replacement_variable, 3 );
	}

	/**
	 * Gets the prefix from a replacement variable name.
	 *
	 * @param string $replacement_variable The replacement variable.
	 *
	 * @return string The prefix of the replacement variable.
	 */
	private function get_prefix( $replacement_variable ) {
		return substr( $replacement_variable, 0, 3 );
	}

	/**
	 * Strips 'desc_' if present, and appends ' description' at the end.
	 *
	 * @param string $label The replacement variable.
	 *
	 * @return string The altered replacement variable name.
	 */
	private function handle_description( $label ) {
		if ( strpos( $label, 'desc_' ) === 0 ) {
			return substr( $label, 5 ) . ' description';
		}

		return $label;
	}

	/**
	 * Creates a label for prefixed replacement variables that matches the format in the editors.
	 *
	 * @param string $replacement_variable The replacement variable.
	 *
	 * @return string The replacement variable label.
	 */
	private function get_label( $replacement_variable ) {
		$prefix = $this->get_prefix( $replacement_variable );
		if ( $prefix === 'cf_' ) {
			return $this->strip_prefix( $replacement_variable ) . ' (custom field)';
		}

		if ( $prefix === 'ct_' ) {
			$label = $this->strip_prefix( $replacement_variable );
			$label = $this->handle_description( $label );
			return ucfirst( $label . ' (custom taxonomy)' );
		}

		if ( $prefix === 'pt_' ) {
			if ( $replacement_variable === 'pt_single' ) {
				return 'Post type (singular)';
			}

			return 'Post type (plural)';
		}

		return '';
	}

	/**
	 * Formats the replacement variables.
	 *
	 * @param string $replacement_variable The replacement variable to format.
	 *
	 * @return array The formatted replacement variable.
	 */
	private function format_replacement_variable( $replacement_variable ) {
		return [
			'name'  => $replacement_variable,
			'value' => '',
			'label' => $this->get_label( $replacement_variable ),
		];
	}

	/**
	 * Set/translate the help texts for the WPSEO standard basic variables.
	 */
	private static function set_basic_help_texts() {
		/* translators: %s: wp_title() function. */
		$separator_description = __( 'The separator defined in your theme\'s %s tag.', 'wordpress-seo' );
		$separator_description = sprintf(
			$separator_description,
			// '<code>wp_title()</code>'
			'wp_title()'
		);

		$replacement_variables = [
			new WPSEO_Replacement_Variable( 'date', __( 'Date', 'wordpress-seo' ), __( 'Replaced with the date of the post/page', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'title', __( 'Title', 'wordpress-seo' ), __( 'Replaced with the title of the post/page', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'parent_title', __( 'Parent title', 'wordpress-seo' ), __( 'Replaced with the title of the parent page of the current page', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'archive_title', __( 'Archive title', 'wordpress-seo' ), __( 'Replaced with the normal title for an archive generated by WordPress', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'sitename', __( 'Site title', 'wordpress-seo' ), __( 'The site\'s name', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'sitedesc', __( 'Tagline', 'wordpress-seo' ), __( 'The site\'s tagline', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'excerpt', __( 'Excerpt', 'wordpress-seo' ), __( 'Replaced with the post/page excerpt (or auto-generated if it does not exist)', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'excerpt_only', __( 'Excerpt only', 'wordpress-seo' ), __( 'Replaced with the post/page excerpt (without auto-generation)', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'tag', __( 'Tag', 'wordpress-seo' ), __( 'Replaced with the current tag/tags', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'category', __( 'Category', 'wordpress-seo' ), __( 'Replaced with the post categories (comma separated)', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'primary_category', __( 'Primary category', 'wordpress-seo' ), __( 'Replaced with the primary category of the post/page', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'category_description', __( 'Category description', 'wordpress-seo' ), __( 'Replaced with the category description', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'tag_description', __( 'Tag description', 'wordpress-seo' ), __( 'Replaced with the tag description', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'term_description', __( 'Term description', 'wordpress-seo' ), __( 'Replaced with the term description', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'term_title', __( 'Term title', 'wordpress-seo' ), __( 'Replaced with the term name', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'searchphrase', __( 'Search phrase', 'wordpress-seo' ), __( 'Replaced with the current search phrase', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'term_hierarchy', __( 'Term hierarchy', 'wordpress-seo' ), __( 'Replaced with the term ancestors hierarchy', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'sep', __( 'Separator', 'wordpress-seo' ), $separator_description ),
			new WPSEO_Replacement_Variable( 'currentdate', __( 'Current date', 'wordpress-seo' ), __( 'Replaced with the current date', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'currentyear', __( 'Current year', 'wordpress-seo' ), __( 'Replaced with the current year', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'currentmonth', __( 'Current month', 'wordpress-seo' ), __( 'Replaced with the current month', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'currentday', __( 'Current day', 'wordpress-seo' ), __( 'Replaced with the current day', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'post_year', __( 'Post year', 'wordpress-seo' ), __( 'Replaced with the year the post was published', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'post_month', __( 'Post month', 'wordpress-seo' ), __( 'Replaced with the month the post was published', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'post_day', __( 'Post day', 'wordpress-seo' ), __( 'Replaced with the day the post was published', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'author_first_name', __( 'Author first name', 'wordpress-seo' ), __( 'Replaced with the first name of the author', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'author_last_name', __( 'Author last name', 'wordpress-seo' ), __( 'Replaced with the last name of the author', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'permalink', __( 'Permalink', 'wordpress-seo' ), __( 'Replaced with the permalink', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'post_content', __( 'Post Content', 'wordpress-seo' ), __( 'Replaced with the post content', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'category_title', __( 'Category Title', 'wordpress-seo' ), __( 'Current or first category title', 'wordpress-seo' ) ),
		];

		foreach ( $replacement_variables as $replacement_variable ) {
			self::register_help_text( 'basic', $replacement_variable );
		}
	}

	/**
	 * Set/translate the help texts for the WPSEO standard advanced variables.
	 */
	private static function set_advanced_help_texts() {
		$replacement_variables = [
			new WPSEO_Replacement_Variable( 'pt_single', __( 'Post type (singular)', 'wordpress-seo' ), __( 'Replaced with the content type single label', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'pt_plural', __( 'Post type (plural)', 'wordpress-seo' ), __( 'Replaced with the content type plural label', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'modified', __( 'Modified', 'wordpress-seo' ), __( 'Replaced with the post/page modified time', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'id', __( 'ID', 'wordpress-seo' ), __( 'Replaced with the post/page ID', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'name', __( 'Name', 'wordpress-seo' ), __( 'Replaced with the post/page author\'s \'nicename\'', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'user_description', __( 'User description', 'wordpress-seo' ), __( 'Replaced with the post/page author\'s \'Biographical Info\'', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'page', __( 'Page', 'wordpress-seo' ), __( 'Replaced with the current page number with context (i.e. page 2 of 4)', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'pagetotal', __( 'Pagetotal', 'wordpress-seo' ), __( 'Replaced with the current page total', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'pagenumber', __( 'Pagenumber', 'wordpress-seo' ), __( 'Replaced with the current page number', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'caption', __( 'Caption', 'wordpress-seo' ), __( 'Attachment caption', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'focuskw', __( 'Focus keyword', 'wordpress-seo' ), __( 'Replaced with the posts focus keyphrase', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'term404', __( 'Term404', 'wordpress-seo' ), __( 'Replaced with the slug which caused the 404', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'cf_<custom-field-name>', '<custom-field-name> ' . __( '(custom field)', 'wordpress-seo' ), __( 'Replaced with a posts custom field value', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'ct_<custom-tax-name>', '<custom-tax-name> ' . __( '(custom taxonomy)', 'wordpress-seo' ), __( 'Replaced with a posts custom taxonomies, comma separated.', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'ct_desc_<custom-tax-name>', '<custom-tax-name> ' . __( 'description (custom taxonomy)', 'wordpress-seo' ), __( 'Replaced with a custom taxonomies description', 'wordpress-seo' ) ),
		];

		foreach ( $replacement_variables as $replacement_variable ) {
			self::register_help_text( 'advanced', $replacement_variable );
		}
	}

	/* *********************** GENERAL HELPER METHODS ************************** */

	/**
	 * Remove the '%%' delimiters from a variable string.
	 *
	 * @param string $text Variable string to be cleaned.
	 *
	 * @return string
	 */
	private static function remove_var_delimiter( $text ) {
		return trim( $text, '%' );
	}

	/**
	 * Add the '%%' delimiters to a variable string.
	 *
	 * @param string $text Variable string to be delimited.
	 *
	 * @return string
	 */
	private static function add_var_delimiter( $text ) {
		return '%%' . $text . '%%';
	}

	/**
	 * Retrieve a post's terms, comma delimited.
	 *
	 * @param int    $id            ID of the post to get the terms for.
	 * @param string $taxonomy      The taxonomy to get the terms for this post from.
	 * @param bool   $return_single If true, return the first term.
	 *
	 * @return string Either a single term or a comma delimited string of terms.
	 */
	public function get_terms( $id, $taxonomy, $return_single = false ) {
		$output = '';

		// If we're on a specific tag, category or taxonomy page, use that.
		if ( ! empty( $this->args->term_id ) ) {
			$output = $this->args->name;
		}
		elseif ( ! empty( $id ) && ! empty( $taxonomy ) ) {
			$terms = get_the_terms( $id, $taxonomy );
			if ( is_array( $terms ) && $terms !== [] ) {
				foreach ( $terms as $term ) {
					if ( $return_single ) {
						$output = $term->name;
						break;
					}
					else {
						$output .= $term->name . ', ';
					}
				}
				$output = rtrim( trim( $output ), ',' );
			}
		}
		unset( $terms, $term );

		/**
		 * Allows filtering of the terms list used to replace %%category%%, %%tag%%
		 * and %%ct_<custom-tax-name>%% variables.
		 *
		 * @api    string    $output    Comma-delimited string containing the terms.
		 * @api    string    $taxonomy  The taxonomy of the terms.
		 */
		return apply_filters( 'wpseo_terms', $output, $taxonomy );
	}

	/**
	 * Gets a taxonomy term hierarchy including the term to get the parents for.
	 *
	 * @return string
	 */
	private function get_term_hierarchy() {
		if ( ! is_taxonomy_hierarchical( $this->args->taxonomy ) ) {
			return '';
		}

		$separator = ' ' . $this->retrieve_sep() . ' ';

		$args = [
			'format'    => 'name',
			'separator' => $separator,
			'link'      => false,
			'inclusive' => true,
		];

		return rtrim(
			get_term_parents_list( $this->args->term_id, $this->args->taxonomy, $args ),
			$separator
		);
	}

	/**
	 * Retrieves the term ancestors hierarchy.
	 *
	 * @return string|null The term ancestors hierarchy.
	 */
	private function retrieve_term_hierarchy() {
		$replacement = null;

		if ( ! empty( $this->args->term_id ) && ! empty( $this->args->taxonomy ) ) {
			$hierarchy = $this->get_term_hierarchy();

			if ( $hierarchy !== '' ) {
				$replacement = esc_html( $hierarchy );
			}
		}

		return $replacement;
	}
}