Description:
------------
isset throws a fatal error when passing a string offset as an array of depth 3 or more when expected behavior would be it returning false.

Reproduce code:
---------------
$string='';
if(isset($string['test']['test']))
        echo "true\n";
else
        echo "false\n";

if(isset($string['test']['test']['test']))
        echo "true\n";
else
        echo "false\n";

Expected result:
----------------
false
false

Actual result:
--------------
false
PHP Fatal error:  Cannot use string offset as an array in /home/anubis/test.php on line 8