ErrorDocument not working with php pages

[Giobibo]

I have in writte a simpy .htacces file with the only line:

ErrorDocument 404 /my404.php

thus if type in the address line of my browser a missing page like:

notexistpage.html

I get correctly my404.php as error page, but if I type in the address
bar of the browser the page:

notexistpage.php

I get the standard apache error page instead of my error page.

In short the "ErrorDocument 404" works only for html pages and fails
for php pages and this without forcing to parse html pages as php
document.

Could anybody help me?

Thank you.