WP_List_Util::sort_callback

	 * @see WP_List_Util::sort()
	 *
	 * @param object|array $a One object to compare.
	 * @param object|array $b The other object to compare.
	 * @return int 0 if both objects equal. -1 if second object should come first, 1 otherwise.
	 */
	private function sort_callback( $a, $b ) {
		if ( empty( $this->orderby ) ) {
			return 0;
		}

		$a = (array) $a;
		$b = (array) $b;

		foreach ( $this->orderby as $field => $direction ) {
			if ( ! isset( $a[ $field ] ) || ! isset( $b[ $field ] ) ) {
				continue;
			}

			if ( $a[ $field ] == $b[ $field ] ) {
				continue;
			}

			$results = 'DESC' === $direction ? array( 1, -1 ) : array( -1, 1 );

			if ( is_numeric( $a[ $field ] ) && is_numeric( $b[ $field ] ) ) {
				return ( $a[ $field ] < $b[ $field ] ) ? $results[0] : $results[1];
			}