Checks whether the given variable is a WordPress Error.

Returns whether `$thing` is an instance of the `WP_Error` class.

function is_wp_error( $thing ) {
	$is_wp_error = ( $thing instanceof WP_Error );

	if ( $is_wp_error ) {
		/**
		 * Fires when `is_wp_error()` is called and its parameter is an instance of `WP_Error`.
		 *
		 * @since 5.6.0
		 *
		 * @param WP_Error $thing The error object passed to `is_wp_error()`.
		 */
		do_action( 'is_wp_error_instance', $thing );
	}

	return $is_wp_error;
}