PHP files not updating

I've started to encounter a weird problem with my server and updating php files. When I edit a php file and save it to the ftp the change can take an entire day to show up, or not at all. If I create a test.php file which intitially has the content "test" and then edit the content and save it the change won't be displayed.

This problem doesn't seem to apply to html files, just php/phtml files.

This started about a month ago.


EDIT:
I just tried restarting the FTP server and that applied the changes made to the php files, BUT further changes don't take effect unless the server is restarted again.

Is there a server-side cache that's been enabled that only gets cleared when the FTP restarts?