PHP - & In Php - It's Not The And Operator

I've been looking through the Joomla docs trying to understand how everything works and I've seen something that I don't understand and cannot find an answer to anywhere on the net.

The basic question is what is the ampersand used for in this conditional he

if (!($mask & 1) && is_string($var)) {

The two in the middle are obviously the AND part of the conditional.  I know it can be used for creating references to variables but I don't think this is what is happening here.  I've seen people using it to test for odd and even numbers too but with no explanation of how it works/what it does.  The full code is below:


function _cleanVar($var, $mask = 0, $type=null)
    {
        // Static input filters for specific settings
        static $noHtmlFilter    = null;
        static $safeHtmlFilter    = null;
 
        // If the no trim flag is not set, trim the variable
                if (!($mask & 1) && is_string($var)) {
            $var = trim($var);
        }
 
        // Now we handle input filtering
                if ($mask & 2)
        {
            // If the allow raw flag is set, do not modify the variable
                        $var = $var;
        }
        elseif ($mask & 4)
        {
            // If the allow html flag is set, apply a safe html filter to the variable
                        if (is_null($safeHtmlFilter)) {
                $safeHtmlFilter = & JFilterInput::getInstance(null, null, 1, 1);
            }
            $var = $safeHtmlFilter->clean($var, $type);
        }
        else
        {
            // Since no allow flags were set, we will apply the most strict filter to the variable
                        if (is_null($noHtmlFilter)) {
                $noHtmlFilter = & JFilterInput::getInstance(/* $tags, $attr, $tag_method, $attr_method, $xss_auto */);
            }
            $var = $noHtmlFilter->clean($var, $type);
        }
        return $var;
    }