Use of undefined constant ƒ - assumed 'ƒ'

If you have this message showing this : 

Warning: Use of undefined constant ƒ - assumed 'ƒ' (this will throw an Error in a future version of PHP) in /home/.../wp-content/plugins/ultimate-sms-notifications/includes/admin/class-woo-usn-custom-post-types.php on line 443

You must upgrade to the latest version of our plugin.