Github Service Hooks

This is the Issue I follow the (The Perfect Work-flow with Git, GitHub, and SSH) of Jeffrey Way on youtube, if you want to watch it this is the link: http://youtu.be/hv4hwgQO9xI

Anyway when I push a commit to a Github Repository my webpage doesn’t update the remote server repository.

In the service Hooks I put http://zazon.mx/github.php

The Script of github.php looks like this:

<?php shell_exec(‘git pull’); ?>

I don’t know why my remote repository doesn’t update with the github one when I push a commit to github.

I use Ubuntu 12.04 with Apache 2, what am I doing bad ?

Of course if I log to shell with my credentials: ssh root@myip and I put git pull I update my remote repository this way.

I apreciate a help thanks