update_metadata_by_mid

update_metadata_by_mid: 這個函式使用後設資料ID更新一個文章、使用者、評論或術語的後設資料。它需要兩個引數，後設資料ID和一個後設資料陣列。

按元ID更新後設資料。

function update_metadata_by_mid( $meta_type, $meta_id, $meta_value, $meta_key = false ) {
	global $wpdb;

	// Make sure everything is valid.
	if ( ! $meta_type || ! is_numeric( $meta_id ) || floor( $meta_id ) != $meta_id ) {
		return false;
	}

	$meta_id = (int) $meta_id;
	if ( $meta_id <= 0 ) {
		return false;
	}

	$table = _get_meta_table( $meta_type );
	if ( ! $table ) {
		return false;
	}

	$column    = sanitize_key( $meta_type . '_id' );
	$id_column = ( 'user' === $meta_type ) ? 'umeta_id' : 'meta_id';

	/**
	 * Short-circuits updating metadata of a specific type by meta ID.
	 *
	 * The dynamic portion of the hook name, `$meta_type`, refers to the meta object type
	 * (post, comment, term, user, or any other type with an associated meta table).
	 * Returning a non-null value will effectively short-circuit the function.
	 *
	 * Possible hook names include:
	 *
	 *  - `update_post_metadata_by_mid`
	 *  - `update_comment_metadata_by_mid`
	 *  - `update_term_metadata_by_mid`
	 *  - `update_user_metadata_by_mid`
	 *
	 * @since 5.0.0
	 *
	 * @param null|bool    $check      Whether to allow updating metadata for the given type.
	 * @param int          $meta_id    Meta ID.
	 * @param mixed        $meta_value Meta value. Must be serializable if non-scalar.
	 * @param string|false $meta_key   Meta key, if provided.
	 */
	$check = apply_filters( "update_{$meta_type}_metadata_by_mid", null, $meta_id, $meta_value, $meta_key );
	if ( null !== $check ) {
		return (bool) $check;
	}

	// Fetch the meta and go on if it's found.
	$meta = get_metadata_by_mid( $meta_type, $meta_id );
	if ( $meta ) {
		$original_key = $meta->meta_key;
		$object_id    = $meta->{$column};

		// If a new meta_key (last parameter) was specified, change the meta key,
		// otherwise use the original key in the update statement.
		if ( false === $meta_key ) {
			$meta_key = $original_key;
		} elseif ( ! is_string( $meta_key ) ) {
			return false;
		}

		$meta_subtype = get_object_subtype( $meta_type, $object_id );

		// Sanitize the meta.
		$_meta_value = $meta_value;
		$meta_value  = sanitize_meta( $meta_key, $meta_value, $meta_type, $meta_subtype );
		$meta_value  = maybe_serialize( $meta_value );

		// Format the data query arguments.
		$data = array(
			'meta_key'   => $meta_key,
			'meta_value' => $meta_value,
		);

		// Format the where query arguments.
		$where               = array();
		$where[ $id_column ] = $meta_id;

		/** This action is documented in wp-includes/meta.php */
		do_action( "update_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

		if ( 'post' === $meta_type ) {
			/** This action is documented in wp-includes/meta.php */
			do_action( 'update_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
		}

		// Run the update query, all fields in $data are %s, $where is a %d.
		$result = $wpdb->update( $table, $data, $where, '%s', '%d' );
		if ( ! $result ) {
			return false;
		}

		// Clear the caches.
		wp_cache_delete( $object_id, $meta_type . '_meta' );

		/** This action is documented in wp-includes/meta.php */
		do_action( "updated_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

		if ( 'post' === $meta_type ) {
			/** This action is documented in wp-includes/meta.php */
			do_action( 'updated_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
		}

		return true;
	}

	// And if the meta was not found.
	return false;
}