The Developer Tutorials blog has a new post covering a very helpful tool when it comes to debugging your sever-side scripts with something light and client-side (but powerful) - the FirePHP project combined with Firebug.
FirePHP is a plugin for Firebug, the web development plugin for Firefox, that allows PHP scripts to talk to a Firebug panel. FirePHP installs alongside Firebug, and provides a simple PHP library to bridge the two. FirePHP provides a window of insight into your PHP applications, with a simple debugging interface that won't interfere with your page content. If you already use Firebug on PHP-powered applications, FirePHP is definitely worth a look.
The FirePHP library installs onto your server and is included, via PHP, into your application. Passing errors back is as simple as using the fb() method to send information out to the Firebug instance waiting on your browser.