github_receive.php

<?php
// Use in the "Post-Receive URLs" section of your GitHub repo.
if ( $_POST['payload'] ) {
  $data = json_decode($_POST['payload']);
  if ($data->ref == 'refs/heads/master') {
    shell_exec( 'cd /home/YOURNAME/webapps/WEBAPPNAME && git pull' );
  }
}
?>