Workaround ISP's redirect of 404 Not Found PHP pages?

Hi, I searched archived messages for this, but didn't find an answer. If this should go to the installation list, please advise (and forgive!). I'm working on a site hosted at 1and1. I want to use a PHP-enabled custom 404 page, but something in their configuration is preventing it from working. My .htaccess file includes the directive: ErrorDocument 404 /not_found.php It works for everything but .php files. If I try to browse a .php file that doesn't exist, the server returns 1and1's default 404 page, not my custom page. After a few rounds of support requests and misinformation, their tech support folks told me this:

PHP is running as a CGI on the shared hosting servers, which is why your .htaccess isn't working for it. You will have to create the error rule within a php.ini file.

Firstly, I can't find anything in their configuration that would suggest what they're doing to acccomplish the redirect. For reference, the phpinfo page (sorry, had to sanitize a few things) can be viewed here: http://www2.jdgcommunications.com/phpexample/phpinfo.html And secondly, how could I incorporate an "error rule" in a local php.ini file? I know how to change settings for error display, reporting and logging, but I haven't been able to find any configuration directives even remotely related to what I'm trying to do. Thanks, Jim