WordPress at a glance
function is not described

is_client_error() WP 1.0

No Hooks.

Return

Nothing (null).

Usage

is_client_error ( $sc );

Code of is client error: wp-includes/rss.php VER 5.1.1

<?php
function is_client_error ($sc) {
	return $sc >= 400 && $sc < 500;
}