ACF fields not effected by strtolower

1

In the below function I can’t get str_replace and strtolower to work. Does anyone have any ideas why not please?

function service_anchor()
{
    $service_title = '{acf_service_group_service_title}';
    $string = str_replace(" ", "-", $service_title);
    return strtolower($string);
}