What does an exclamaton mark in front of a variable mean? And how is it being used in this piece of code?

EDIT: From the answers so far I suspect that I also should mention that this code is in a function where one of the parameters is $mytype ....would this be a way of checking if $mytype was passed? - Thanks to all of the responders so far.

 $myclass = null;

    if ($mytype == null && ($PAGE->pagetype <> 'site-index' && $PAGE->pagetype <>'admin-index')) {
        return $myclass;
    }
    elseif ($mytype == null && ($PAGE->pagetype == 'site-index' || $PAGE->pagetype =='admin-index')) {
        $myclass = ' active_tree_node';
        return $myclass;
    }
    elseif (!$mytype == null && ($PAGE->pagetype == 'site-index' || $PAGE->pagetype =='admin-index')) {
        return $myclass;
    }`