Xdebug (A PHP extension for debugging PHP scripts)

The Xdebug extension helps you debugging your script by providing a lot of
valuable debug information.  The debug information that Xdebug can provide
includes the following: stack traces and function traces in error messages,
memory allocation and protection for infinite recursions. Xdebug also
provides profiling information for PHP, scripts, code coverage analysis and
capabilities to debug PHP scripts interactively with a debug client.

This requires libedit.

Once you've installed xdebug, uncomment the following line in /etc/php/xdebug.ini:
; zend_extension = "/usr/lib/php/extensions/xdebug.so"

Restart your webserver if you're using php with it, and as a recomendation
from xdebug's README: "Write a PHP page that calls "phpinfo();" Load it
in a browser and look for the info on the xdebug module. If you see it,
you have been successful!"

Note that some of xdebug's functionality may be dependent on php core
configuration settings.