boolean variable values in PHP to javascript implementation

I've run into an odd issue in a PHP script that I'm writing-- I'm sure there's an easy answer but I'm not seeing it.

I'm pulling some vars from a DB using PHP, then passing those values into a Javascript that is getting built dynamically in PHP. Something like this:

$myvar = (bool) $db_return->myvar;

$js = "<script type=text/javascript>
        var myvar = " . $myvar . ";
        var myurl = 'http://someserver.com/ajaxpage.php?urlvar=myvar';
       </script>";

The problem is that if the boolean value in the DB for "myvar" is false, then the instance of myvar in the $js is null, not false, and this is breaking the script.

Is there a way to properly pass the value false into the myvar variable?

Thanks!