Description:
------------
This is probably related to refcount of $this that happens only when the object is invoked as a closure:

http://3v4l.org/7PZip

When there is an assignment the same problem does not occurs:

http://3v4l.org/deLUQ

Test script:
---------------
<?php

class X {
    function __invoke() {
        var_dump($this);
    }
}

(new X)();

Expected result:
----------------
object(X)#1 (0) {
}

Actual result:
--------------
*RECURSION*