PHP: Detecting a valid request via a pass key set in a header

secure-php-cron.php

<?php
	if('my-agent' !== $_SERVER['HTTP_USER_AGENT'] or
	   $_SERVER['SERVER_ADDR'] !== $_SERVER['REMOTE_ADDR'] or
	   !isset($_SERVER['HTTP_TASK_KEY']) or
	   empty($_SERVER['HTTP_TASK_KEY']) or
	   'my-secret-key' !== $_SERVER['HTTP_TASK_KEY']) {
		print 'You do not have permission to execute this script!';
		exit();
	}
	print 'Executing operation.';
          //insert your processing script here
?>