According to xdebug manual:
xdebug.remote_enable Type: boolean, Default value: 0
controls whether Xdebug should try to contact a debug client which is
listening on the host and port as set with the settings
xdebug.remote_host and xdebug.remote_port. If a connection can not be
established the script will just continue as if this setting was 0.
I order to setup remote debugging, you should change your
/etc/php5/apache2/conf.d/20-xdebug.ini and add this line:
xdebug.remote_enable = 1
You should setup xdebug.remote_port and xdebug.remote_host too.
Read this link for more information on Remote Debugging