PHP developers working on Debian have one more reason to be happy with their choice of linux distributions - as Derick Reathans notes, XDebug has been added as an apt-getable package to the Debian systems.
Since a few days, there is a new package in Debian: php5-xdebug. After a few years of talking licenses, due to the help of Martin Meredith and Francois Marier Xdebug can finally be installed with apt-get.
XDebug is a debugging and profiling tool that adds additional information (more than just the usual "error on line..." sort of thing) to the output of your PHP scripts. It also adds more complex functionality like profiling and code coverage reports run on your applications.