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.

Once you've installed xdebug, uncomment the following line in
/etc/php.d/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.