Vim is able to auto-complete syntax for some of the most popular programming languages, but it’s not enabled by default.

While in command mode, enter the following 

:set omnifunc=phpcomplete#CompletePHP

To start using PHP auto completion, press the Ctrl + x followed by Ctrl + o keys while writing some PHP functions.

Below is an example screenshot when pressing Ctrl + x followed by Ctrl + o after typing dat

Do note the function description provided at the top of the screen.