This sometimes happens if a PHP script exits unexpectedly - I had this happen while I was trying to generate a spreadsheet using PHPExcel. You might want to check your php.ini settings to make sure all error reporting is switched on or maybe try uploading a very basic test script such as
to see what happens. Assuming you have shell access to the server you can run PHP at the command line to see what's going on and make sure it works from the interpreter, and you might also want to check your server logs for web requests and upgrade activity. It would also help if you could both tell us what 3rd party PHP software you're running to power your sites.