Php error: Use of undefined constant W3TC_POWERED_BY

Use of undefined constant W3TC_POWERED_BY – assumed ‘W3TC_POWERED_BY’ 1 wp-content/plugins/google-webfont-optimizer/google-webfont-optimizer.php:342

Changed the above mentioned line to include the backticks:

/**
         * Check User Agent
         */
        if (isset($_SERVER['HTTP_USER_AGENT']) && stristr($_SERVER['HTTP_USER_AGENT'], 'W3TC_POWERED_BY') !== false) {
            return false;
        }

`

https://wordpress.org/plugins/google-webfont-optimizer/