php github webhook没有在服务器上执行

I am trying to push changes to server with github webhook, with the following php script:

<?php
    shell_exec("cd /var/www/repo.io/ && git pull");

?>

Push is detected on github, but no action is being triggered on server. Also, I got no permission or any other kind of error on the server. So, php script is not executed at all from the web. But when I am inside server, through CLI - it works. What can be the problem ?

Thanks