sudo apt-get install php5-xdebug

for configuration, the file

sudo vim /etc/php5/apache2/conf.d/20-xdebug.ini

open and enter the following contents

zend_extension=xdebug.so
xdebug.idekey='fossydebug'
xdebug.remote_autostart=0
xdebug.remote_enable=1
xdebug.remote_host=10.0.2.2
xdebug.remote_port=9000

Just to be on the safe side, just restart the apache server so you are sure that the config is read

sudo service apache2 restart

From now on the apache server will send requests with activated Debug-Cookie or referrign information to the 10.0.2.2:9000. The IP 10.0.2.2 is the IP of the host system on which the IDE actually runs.

For this some extensions for multiple browsers are available. For Firefox for example "The easiest XDebug 2.0".

IDE should listen on port 9000 to incoming Debug-Connection-Requests.