PHP files downloading, rather than parsing

[John Lyon]

OK. Stupid n00b question, but I'm stumped, and I don't even know what
questions to ask, or what info to include.

I've got a drupal install that is now broken - the PHP files are not being
parsed; FireFox wants to download them instead.

Using Webmin, it looks to me like the PHP mime-type is configured correctly,
so I'm not sure what's going on.

application/x-httpd-php     phtml pht php
application/x-httpd-php-source     phps
application/x-httpd-php3     php3
application/x-httpd-php3-preprocessed     php3p
application/x-httpd-php4     php4

Here's the php stuff installed:

libapache2-mod-php4     K-O     server-side, HTML-embedded scripting
  language (apache 2.0 module)
php4     P-T     server-side, HTML-embedded scripting language
  (meta-package)
php4-cli     P-T     command-line interpreter for the php4 scripting
  language
php4-common     P-T     Common files for packages built from the php4 source
php4-mcrypt     P-T     MCrypt module for php4
php4-mysql     P-T     MySQL module for php4
phpmyadmin     P-T     set of PHP-scripts to administrate MySQL over the WWW

Apache is serving up pages, and indicates:
Apache/2.0.54 (Ubuntu) PHP/4.4.0-3ubuntu1 Server at 10.0.0.245 Port 80

A shove in the right direction would be much appreciated.

John Lyon
n00b in Austin