Xdebug

On Ubuntu it's easy, on XAMPP it's not too hard either.

For Ubuntu:

sudo apt-get install php-xdebug

Check phpinfo now has mention of xdebug.

Make sure your php mods-available folder contains xdebug.ini file and this file contains below line:

zend_extension=xdebug.so

Default xdebug location: /etc/php/7.0/mods-available/xdebug.ini

Now, add below lines at the bottom of your php.ini file:

[XDebug]
xdebug.remote_enable=1
xdebug.remote_autostart=1

Finally restart apache.

sudo service apache2 restart

Add in the theeasiestxdebug firefox extension.

In VS Code add the PHP Debug extension, restart VS Code