CentOS 5 :: Looking For Apache Performance Monitoring?

In Short: I want something that will show me which files and associated requested URLs are causing the highest load on our web server.

Verbose: We already monitor our Apache web server using Cacti, Nagios, and of course traditional nix commands, but I am looking for something specifically to take an Apache process ID and tie that to a users request for a specific file and URL. The great thing about something like this is that it would be able to show me the worse performing PHP files. Special bonus if it can store this data over time for reports.

I have been googling around for something like this but can't seem to find it. Bonus if it exists in Cacti or Nagios already and I'm just to blind. I started writing my own PERL script to do this, but have limited time to devote to this at work so if a solution already exists I'm game, else I'll just have to write it myself. I'm also worried about my own scripted solution in that it won't get it write because it will be leveraging the output of a specialzed apache log that records the PID and then doing a look up with ps aux looking for that PID.