apache not handling 404 errors for .php pages

I have custom 404 error handling setup on my linux apache box, however, there's a problem with files that have a PHP extension. it seems that apache itself does not check for the existence of the requested .php file and fires up PHP no matter what. if PHP does not find the requested file, it'll bail out with that dorky looking "Internal Server Error" *NOT* the custom error handler that apache uses for non-php files.... example: "notexists.html" and "notexists.php" both do not exist http://server.com/notexists.html will show me the custom 404 page http://server.com/notexists.php will show me the "Internal Server Error", becuase apache fired up PHP and passed in "notexists.php" without checking for the existence of "notexists.php" first. A possible solution that comes to mind is to auto_prepend a script that will check for the existance of the file that PHP is attempting to execute, and die gracefully if it does not exist on the server.... but that would be a bad kludge hack (if it even works at all) anyone have a more elegant solution?