Hi Ian & Jason,
When I go to Admin.php and enter the test username and password, instead of being directed to admin_search.php, I get a download window asking what I want to do with the file, - open it with a program, or save the file.
This is the behaviour I would expect to see if PHP was disabled somehow, but as your other PHP page reads OK, it seems strange that this happens. I haven't seen this before. Maybe it's a PHP version issue?