$_GET keys whitelist

gistfile1.php

<?php

if(!validate_request()) {
	die('you requested data from a $_GET key that is not whitelisted.');
}

echo 'Your request is valid';

function validate_request($additional_keys = array()) {

	$keys = array('controller', 'action', 'page');

	if(count($additional_keys)) {
	    $keys += $additional_keys;	
	}

	return !array_filter($_GET, function($data) use ($keys) {
	    return (!in_array($data, $keys));
	}, ARRAY_FILTER_USE_KEY);
}