changing your code from php short tag to <?php and then adding error reporting and display errors as such displays
<?php
error_reporting(E_ALL);
ini_set('display_errors', '1');
Warning: require_once(/var/www/vhosts/worldwide-gaming.com/httpdocs/squirrelcart/pre_common.php): failed to open stream: No such file or directory in /var/www/html/ProdFeedGen.php on line 6
Fatal error: require_once(): Failed opening required '/var/www/vhosts/worldwide-gaming.com/httpdocs/squirrelcart/pre_common.php' (include_path='.:/usr/share/pear:/usr/share/php') in /var/www/html/ProdFeedGen.php on line 6
and does not print code to screen.
These errors show for me due to the fact I am not using a full url to get to your require once file.