bitbucket webhook权限被拒绝

Question

I created a php file to run git pull when there is a git push. added it to bitbucket webook.

But Im running into permission issue. I checked the Owner/Group of my /var/www/html/ is root root. and the /var/www/html/.git/ is also root root. I am running ubuntu 14.04 and apache2

Error Log:

error: cannot open .git/FETCH_HEAD: Permission denied

Webhook file:

<?php
    $output = shell_exec('git pull');
    echo "<pre>$output</pre>";
?>