Description:
------------
When using a custom class as a callback object, the array_map (and possibly other methods) call the method outside of the object's context.

I was using this as the __invoke() method to test:

        if (!isset($this)) {
            die('Outside of object context.');
        } else {
            return $this->val;
        }

This may be the best way to setup the PHP unit test script.

Again I did not test this on other callback functions, but it very easily could be happening on those as well.

Reproduce code:
---------------
<?php
class Callback
{
    protected $val = 'hello, world';
    
    public function __invoke()
    {
        return $this->val;
    }
}

$cb = new Callback();

// straight callback
if ($cb() != 'hello, world') {
    die('error');
}

// array_map
$a = array(1, 2, 3);
$b = array_map($cb, $a);
print_r($b);
?>

Expected result:
----------------
Array
(
    [0] => hello, world
    [1] => hello, world
    [2] => hello, world
)


Actual result:
--------------
PHP Fatal error:  Using $this when not in object context in /srv/www/test/closure.php on line 8