Install xDebug:
apt-get install php5-xdebug

Find xDebug location by:
locate xdebug.so
, it will return something like /usr/lib/php5/20121212/xdebug.so

Add following lines into php.ini:
  zend_extension="/usr/lib/php5/20121212/xdebug.so"
  xdebug.remote_autostart=on
  xdebug.remote_enable=1
  xdebug.remote_handler=dbgp
  xdebug.remote_mode=req
  xdebug.remote_host=127.0.0.1
  xdebug.remote_port=9000