<?php
    $foo = 'foo';
    function showvar( $bar = $foo )
    {
        //global $foo
        //doesn't work either
        echo $bar;
    }
    showvar();
    showvar('bar');
?>

gives me a parse error on line 3:(

also:

<?php
    $foo = 'foo';
    function returnvar()
    {
        global $foo;
        return $foo;
    }

    function whatisvar( $bar = returnvar() )
    {
        echo $bar;
    }
?>

gives me this:
Parse error: parse error, expecting `')'' in c:/path/to/script/test.php on line 9

Should work, shouldn't it?