MySQL is used by IMP for three things: user preference storage, address book storage, and session management. All of these are done, if memory serves, in the horde database, so changes made to other databases shouldn't make a difference.
To check if PHP is working, create a page called test.php containing the following line:
phpinfo(); ?>
...and place it somewhere where your web server can read it. Then browse to that file with your favorite browser. If you get an informative page, PHP is working. If you don't, it isn't, at least for that location.