Any suggestions for tools to monitor page load times/errors and other performance metrics for a PHP application?

I am aware of the FireBug and YSlow tools, but this is for more server monitoring.

+4  A: 

There is the classic 'ab' (apachebench) program. More power comes from JMmeter. For server health, I recommend Munin, which can painlessly capture data from several systems and aggregate it on one page.

Yann Ramin
+1  A: 

Try Nagios, it's the default tool to monitor servers. You can write plugins to report just about any data.

Ryan Doherty
+1  A: 

For profiling your code, there's Xdebug. Doing regression testing with Siege can also be quite useful.

Kristian J.
+1  A: 

You can also try httperf. It's a very flexible tool and if you want to test how your application and webserver can deal with various traffic loads you should definitely give it a go.

Robert Smith