functions.php

<?php
if ( ! defined( 'ABSPATH' ) ) {
	return;
}

/**
 * Retrieve Ghost metadata for the specified object.
 *
 * @param int    $object_id   ID of the object metadata is for
 * @param string $meta_key    Optional. Metadata key. If not specified, retrieve all metadata for
 *                            the specified object.
 * @param bool   $single      Optional, default is false.
 *                            If true, return only the first value of the specified meta_key.
 *                            This parameter has no effect if meta_key is not specified.
 *
 * @return mixed Single metadata value, or array of values
 *
 * @see get_metadata()
 */
function get_ghost_meta( $object_id, $meta_key = '', $single = false ) {

	return get_metadata( 'ghost', $object_id, $meta_key, $single );

}

/**
 * Add Ghost metadata for the specified object.
 *
 * @param int    $object_id  ID of the object metadata is for
 * @param string $meta_key   Metadata key
 * @param mixed  $meta_value Metadata value. Must be serializable if non-scalar.
 * @param bool   $unique     Optional, default is false.
 *                           Whether the specified metadata key should be unique for the object.
 *                           If true, and the object already has a value for the specified metadata key,
 *                           no change will be made.
 *
 * @return int|false The meta ID on success, false on failure.
 *
 * @see add_metadata()
 */
function add_ghost_meta( $object_id, $meta_key, $meta_value, $unique = false ) {

	return add_metadata( 'ghost', $object_id, $meta_key, $meta_value, $unique );

}

/**
 * Update Ghost metadata for the specified object. If no value already exists for the specified object
 * ID and metadata key, the metadata will be added.
 *
 * @param int    $object_id    ID of the object metadata is for
 * @param string $meta_key     Metadata key
 * @param mixed  $meta_value   Metadata value. Must be serializable if non-scalar.
 * @param mixed  $prev_value   Optional. If specified, only update existing metadata entries with
 *                             the specified value. Otherwise, update all entries.
 *
 * @return int|bool Meta ID if the key didn't exist, true on successful update, false on failure.
 *
 * @see update_metadata()
 */
function update_ghost_meta( $object_id, $meta_key, $meta_value, $prev_value = '' ) {

	return update_metadata( 'ghost', $object_id, $meta_key, $meta_value, $prev_value );

}

/**
 * Delete Ghost metadata for the specified object.
 *
 * @param int    $object_id  ID of the object metadata is for
 * @param string $meta_key   Metadata key
 * @param mixed  $meta_value Optional. Metadata value. Must be serializable if non-scalar. If specified, only delete
 *                           metadata entries with this value. Otherwise, delete all entries with the specified
 *                           meta_key. Pass `null, `false`, or an empty string to skip this check. (For backward
 *                           compatibility, it is not possible to pass an empty string to delete those entries with an
 *                           empty string for a value.)
 * @param bool   $delete_all Optional, default is false. If true, delete matching metadata entries for all objects,
 *                           ignoring the specified object_id. Otherwise, only delete matching metadata entries for
 *                           the specified object_id.
 *
 * @return bool True on successful delete, false on failure.
 *
 * @see delete_metadata()
 */
function delete_ghost_meta( $object_id, $meta_key, $meta_value = '', $delete_all = false ) {

	return delete_metadata( 'ghost', $object_id, $meta_key, $meta_value, $delete_all );

}