js-identifiers/check.php

<?php header('content-type: text/html; charset=utf-8');

include 'is_valid_callback.function.php';

# Empty
if(empty($_GET['callback']))
	exit('<span style="color:#777">not set</span>');

# Valid
if(is_valid_callback($_GET['callback']))
	exit('<span style="color:#070">valid</span>');

# Not valid
else
	exit('<span style="color:#900">not valid</span>');