Custom error 500 page

I've managed to redirect to a custom php file to display the error 404 page via the configuration information I've set in the apache2 sites-available "website.conf" file, but the 500 page always displays a default browser 500 error page and not the one i've actually redirected to...

The PHP code takes the number of the "id" code posted to it and generates a large error number on the page and then displays the corresponding error's description...

ErrorDocument 404 /error/error.php?id=404
ErrorDocument 500 /error/error.php?id=500

If the 404 error redirects ok, any ideas why the 500 is not working and how I can fix it?