We've been Beta testers for Zend's new software suite, Zend Canaveral, and I must say it ROCKS.
In particular, is the PHP monitoring section. It alerts you whenever scripts crash, php pages load slowly, etc...
Then when you view the event, you can view exactly what happened, what data was passed to which functions, get and post data to the page, includes, timings, slow queries, etc...
I'm sure they have a much more thorough explanation of the product on their site (it's out of Beta and now officially released), but take it from someone who's been using PHP for approx 6 years now: you want this product
Not to mention it includes their Performance Suite, which has code acceleration, compression and caching tools to help speed things up on your site.