The above error in Google Merchant Center API appear if you are using PHP 5.2 or less. The error is because PHP 5.2 or lower does not recognizes const variable outside the class. PHP 5.3 solves that problem.
To upgrade your PHP version, go to Control Panel and select PHP config. There you may have options to change or upgrade your PHP version. If not not contact your web host.
Const PHP Decumentation. Alternative you can change all Const by define(‘constant’,’value’).