http://www.apaddedcell.com/easy-php-debugging-ubuntu-using-xdebug-and-vim

tldr;

1. Add ?XDEBUG_SESSION_START=1 to the end of the url

2. in vim press F5 and quickly switch to the browser and hit F5 to refresh.

3. switch back to vim, set breakpoint and hit F5 to reach it.