Substr - Can't use value return in parse context

I am trying to create an IF function with two conditions however, I am getting parse error "can't use value return in parse context". Any ideas please?

    $youlink = get_field('fl_tube');               
    if (strlen ($youlink) < 50 && substr($youlink, 0, 23) = "https://www.youtube.com"; ) {
some code here...    
    }