404 error checker

Hi, Im rather a novice at PHP, so I appologize if this question is rather basic. Also, to be sure, I have looked through the PHP site and archives for answers to this question with no luck. Sorry if it is repetative. I am writing a script that will send me an email every time a 404 error is returned for an http request. So far I have the script working so that it will email me and inform me of the URL of the page that has the offending link by using the HTTP_REFERER environment variable. I would also like to find out what the requested page URL is. I have checked phpinfo() for info on other Apache enviro variables, but can't seem to find anything that would work. How can I do this? Is it possible? Thank you, Michael