Xdebug is a php source code debugger. To enable it, create the file /etc/php5/mods-available/xdebug.ini, with the following contents:
If I do not need Xdebug, and PHP performance matters, I do not enable it. With PHP 5.5.9 execution time of performance.php is reduced from more than 12 seconds to less than 3 seconds when xdebug is disabled. Try it yourself! To disable XDebug after it has been enabled, delete /etc/php5/mods-available/xdebug.ini and restart apache, sudo service apache2 restart.
I use the script install-xdebug.sh to install and configure Xdebug.