replacement = null;

		if ( ! empty( $this->args->ID ) ) {
			$replacement = $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 = mysql2date( get_option( 'date_format' ), $this->args->post_modified, true );
		}

		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 = $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 null|string
	 */
	private function retrieve_user_description() {
		$replacement = null;

		$user_id     = $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() {
		$replacement = ! empty( $this->args->post_author ) ? $this->args->post_author : get_query_var( 'author' );

		return $replacement;
	}

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

	/**
	 * Create a variable help text table.
	 *
	 * @param string $type Either 'basic' or 'advanced'.
	 *
	 * @return string Help text table.
	 */
	private static function create_variable_help_table( $type ) {
		if ( ! in_array( $type, array( 'basic', 'advanced' ), true ) ) {
			return '';
		}

		$table = '
			<table class="yoast_help yoast-table-scrollable">
			<thead>
				<tr>
					<th scope="col">' . esc_html__( 'Label', 'wordpress-seo' ) . '</th>
					<th scope="col">' . esc_html__( 'Variable', 'wordpress-seo' ) . '</th>
					<th scope="col">' . esc_html__( 'Description', 'wordpress-seo' ) . '</th>
				</tr>
			</thead>
			<tbody>';

		foreach ( self::$help_texts[ $type ] as $replacement_variable ) {
			$table .= '
				<tr>
					<td class="yoast-variable-label">' . esc_html( $replacement_variable->get_label() ) . '</td>
					<td class="yoast-variable-name">%%' . esc_html( $replacement_variable->get_variable() ) . '%%</td>
					<td class="yoast-variable-desc">' . esc_html( $replacement_variable->get_description() ) . '</td>
				</tr>';
		}

		$table .= '
			</tbody>
			</table>';

		return $table;
	}

	/**
	 * Create the help text table for the basic variables for use in a help tab.
	 *
	 * @return string
	 */
	public static function get_basic_help_texts() {
		return self::create_variable_help_table( 'basic' );
	}

	/**
	 * Create the help text table for the advanced variables for use in a help tab.
	 *
	 * @return string
	 */
	public static function get_advanced_help_texts() {
		return self::create_variable_help_table( 'advanced' );
	}

	/**
	 * 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, array( '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_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( array( $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 ), array( $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 = array( '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 array(
			'name'  => $replacement_variable,
			'value' => '',
			'label' => $this->get_label( $replacement_variable ),
		);
	}

	/**
	 * Retrieves the custom field names as an array.
	 *
	 * @link WordPress core: wp-admin/includes/template.php. Reused query from it.
	 *
	 * @return array The custom fields.
	 */
	private function get_custom_fields() {
		global $wpdb;

		/**
		 * Filters the number of custom fields to retrieve for the drop-down
		 * in the Custom Fields meta box.
		 *
		 * @since 2.1.0
		 *
		 * @param int $limit Number of custom fields to retrieve. Default 30.
		 */
		$limit  = apply_filters( 'postmeta_form_limit', 30 );
		$sql    = "SELECT DISTINCT meta_key
			FROM $wpdb->postmeta
			WHERE meta_key NOT BETWEEN '_' AND '_z'
			HAVING meta_key NOT LIKE %s
			ORDER BY meta_key
			LIMIT %d";
		$fields = $wpdb->get_col( $wpdb->prepare( $sql, $wpdb->esc_like( '_' ) . '%', $limit ) );

		if ( is_array( $fields ) ) {
			return array_map( array( $this, 'add_custom_field_prefix' ), $fields );
		}

		return array();
	}

	/**
	 * Adds the cf_ prefix to a field.
	 *
	 * @param string $field The field to prefix.
	 *
	 * @return string The prefixed field.
	 */
	private function add_custom_field_prefix( $field ) {
		return 'cf_' . $field;
	}

	/**
	 * Gets the names of the custom taxonomies, prepends 'ct_' and 'ct_desc', and returns them in an array.
	 *
	 * @return array The custom taxonomy prefixed names.
	 */
	private function get_custom_taxonomies() {
		$args              = array(
			'public'   => true,
			'_builtin' => false,
		);
		$output            = 'names';
		$operator          = 'and';
		$custom_taxonomies = get_taxonomies( $args, $output, $operator );

		if ( is_array( $custom_taxonomies ) ) {
			$ct_replace_vars = array();
			foreach ( $custom_taxonomies as $custom_taxonomy ) {
				array_push( $ct_replace_vars, 'ct_' . $custom_taxonomy, 'ct_desc_' . $custom_taxonomy );
			}
			return $ct_replace_vars;
		}

		return array();
	}

	/**
	 * 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 = array(
			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( 'sep', __( 'Separator', 'wordpress-seo' ), $separator_description ),
		);

		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 = array(
			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 number', '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 $string Variable string to be cleaned.
	 *
	 * @return string
	 */
	private static function remove_var_delimiter( $string ) {
		return trim( $string, '%' );
	}

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

	/**
	 * 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 ( is_category() || is_tag() || is_tax() ) {
			$term   = $GLOBALS['wp_query']->get_queried_object();
			$output = $term->name;
		}
		elseif ( ! empty( $id ) && ! empty( $taxonomy ) ) {
			$terms = get_the_terms( $id, $taxonomy );
			if ( is_array( $terms ) && $terms !== array() ) {
				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 );
	}
} /* End of class WPSEO_Replace_Vars */