Using a PHP function in Twig

I have a PHP code to add a new class for my Twig template in my common controller in: "opencart\htdocs\catalog\controller\common\cart.php"

The code should check if the Device is Mobile or not.

function onStart(){    // Anonymous Class only working on PHP7    $this['code'] = new class {       public function MobileDetect() {    return preg_match("/(android|avantgo|blackberry|bolt|boost|cricket|docomo|fone|hiptop|mini|mobi|palm|phone|pie|tablet|up\.browser|up\.link|webos|wos)/i", $_SERVER["HTTP_USER_AGENT"]);}    };}

But now I don´t know how to address that function correctly from my twig side at:

opencart\htdocs\catalog\view\theme\default\template\common\cart.twig

I tried something like this, but it didn´t seem to work:

{% if code.MobileDetect() is defined %}

If a device is mobile I want to use a completely different HTML construct.