Description:
------------
There is a problem when I test if an array key exists on a string with the function isset().

isset return true but $foo is not an array and error_reporting doesn't display an error.

Reproduce code:
---------------
$foo = 'foo';

var_dump($foo);
echo '<br />';
var_dump(isset($foo['bar']));

Expected result:
----------------
string(3) "foo"
bool(false)

Actual result:
--------------
string(3) "foo"
bool(true)