update_metadata( string $meta_type, int $object_id, string $meta_key, mixed $meta_value, mixed $prev_value = '' )

function update_metadata($meta_type, $object_id, $meta_key, $meta_value, $prev_value = '') {
	global $wpdb;

	if ( ! $meta_type || ! $meta_key || ! is_numeric( $object_id ) ) {
		return false;
	}

	$object_id = absint( $object_id );
	if ( ! $object_id ) {
		return false;
	}

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

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

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

	// expected_slashed ($meta_key)
	$raw_meta_key = $meta_key;
	$meta_key = wp_unslash($meta_key);
	$passed_value = $meta_value;
	$meta_value = wp_unslash($meta_value);
	$meta_value = sanitize_meta( $meta_key, $meta_value, $meta_type, $meta_subtype );

	/**
	 * Filters whether to update metadata of a specific type.
	 *
	 * The dynamic portion of the hook, `$meta_type`, refers to the meta
	 * object type (comment, post, term, or user). Returning a non-null value
	 * will effectively short-circuit the function.
	 *
	 * @since WP-3.1.0
	 *
	 * @param null|bool $check      Whether to allow updating metadata for the given type.
	 * @param int       $object_id  Object ID.
	 * @param string    $meta_key   Meta key.
	 * @param mixed     $meta_value Meta 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.
	 */
	$check = apply_filters( "update_{$meta_type}_metadata", null, $object_id, $meta_key, $meta_value, $prev_value );
	if ( null !== $check )
		return (bool) $check;

	// Compare existing value to new value if no prev value given and the key exists only once.
	if ( empty($prev_value) ) {
		$old_value = get_metadata($meta_type, $object_id, $meta_key);
		if ( count($old_value) == 1 ) {
			if ( $old_value[0] === $meta_value )
				return false;
		}
	}

	$meta_ids = $wpdb->get_col( $wpdb->prepare( "SELECT $id_column FROM $table WHERE meta_key = %s AND $column = %d", $meta_key, $object_id ) );
	if ( empty( $meta_ids ) ) {
		return add_metadata( $meta_type, $object_id, $raw_meta_key, $passed_value );
	}

	$_meta_value = $meta_value;
	$meta_value = maybe_serialize( $meta_value );

	$data  = compact( 'meta_value' );
	$where = array( $column => $object_id, 'meta_key' => $meta_key );

	if ( !empty( $prev_value ) ) {
		$prev_value = maybe_serialize($prev_value);
		$where['meta_value'] = $prev_value;
	}

	foreach ( $meta_ids as $meta_id ) {
		/**
		 * Fires immediately before updating metadata of a specific type.
		 *
		 * The dynamic portion of the hook, `$meta_type`, refers to the meta
		 * object type (comment, post, term, or user).
		 *
		 * @since WP-2.9.0
		 *
		 * @param int    $meta_id    ID of the metadata entry to update.
		 * @param int    $object_id  Object ID.
		 * @param string $meta_key   Meta key.
		 * @param mixed  $meta_value Meta value.
		 */
		do_action( "update_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

		if ( 'post' == $meta_type ) {
			/**
			 * Fires immediately before updating a post's metadata.
			 *
			 * @since WP-2.9.0
			 *
			 * @param int    $meta_id    ID of metadata entry to update.
			 * @param int    $object_id  Object ID.
			 * @param string $meta_key   Meta key.
			 * @param mixed  $meta_value Meta value.
			 */
			do_action( 'update_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
		}
	}

	$result = $wpdb->update( $table, $data, $where );
	if ( ! $result )
		return false;

	wp_cache_delete($object_id, $meta_type . '_meta');

	foreach ( $meta_ids as $meta_id ) {
		/**
		 * Fires immediately after updating metadata of a specific type.
		 *
		 * The dynamic portion of the hook, `$meta_type`, refers to the meta
		 * object type (comment, post, term, or user).
		 *
		 * @since WP-2.9.0
		 *
		 * @param int    $meta_id    ID of updated metadata entry.
		 * @param int    $object_id  Object ID.
		 * @param string $meta_key   Meta key.
		 * @param mixed  $meta_value Meta value.
		 */
		do_action( "updated_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

		if ( 'post' == $meta_type ) {
			/**
			 * Fires immediately after updating a post's metadata.
			 *
			 * @since WP-2.9.0
			 *
			 * @param int    $meta_id    ID of updated metadata entry.
			 * @param int    $object_id  Object ID.
			 * @param string $meta_key   Meta key.
			 * @param mixed  $meta_value Meta value.
			 */
			do_action( 'updated_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
		}
	}

	return true;
}