Clicking ‘save’ just gets open PHP file dialog

Running Wp 2.2.3.

If I create a new post and click either ‘save and continue’ or just ‘save,’ all that happens is a pop-up dialog asking me if I’d like to open or download a file called post.php.

If I do save the file it has 0kb.

This behaviour started a couple hours ago. Is it something to do with PHP on the server, perhaps? Like, is it running? As far as I can tell, it is. PHP version is 5.2.3.

It’s only these two buttons that produce this problem and it has me totally stumped.

Any suggestions on how to troubleshoot? Thanks.