In this post on Bitacle.org, they look at a 5 minute approach to finding some of the more common issues with PHP web applications.

Detecting and correcting problems with applications at early stages is an important role of the server manager. Unfortunately, not all errors are detected at the testing stages. Even more unfortunate is the fact that most errors go undetected because they are usually triggered when a certain set of criteria is met.

Since all you have is 5 minutes (which is one of the tenets of this Server management series, and quite possibly the only simple truth in your case), in this installment, we'll unlock the secret of server log foraging.

They mainly make use of grep, a very handly unix command-line tool, to parse through the server logs for answers. Combine that with upping the error reporting level inside of PHP itself, and you should be able to track down most of the problems you'd have. They also include a sample situation or two to watch out for specifically.