Description:
------------
Let's say we have a member variable $foo, which will hold something callable. 

We then assign to $foo a closure, say, function($x){ return $x; }:

$this->foo = function($x) { return $x; };

Then, we assign to $foo a new closure, which brings into a scope another closure:

$another_f = function($x) { return $x; };
$this->foo = function() use ($another_f) {
  $args = func_get_args();
  call_user_func_array( $this->foo, $args ); // Crash occurs here
  call_user_func_array( $another_f, $args );
};

The PHP process will crash on the line where it calls $this->foo as a function. Probably recurses into an infinite loop.

Test script:
---------------
https://gist.github.com/BeerendLauwers/c28585ce4868ab42d2f9

Expected result:
----------------
The value returned from $t->apply() should return TRUE.

Actual result:
--------------
PHP process crashes. As a workaround, you can assign $this->foo to a placeholder variable and bring it into scope into the closure via use, which will bind it correctly:

$placeholder = $this->foo;
$another_f = function($x) { return $x; };
$this->foo = function() use ($another_f,$placeholder) {
  $args = func_get_args();
  call_user_func_array( $placeholder, $args );
  call_user_func_array( $another_f, $args );
};

Perhaps make it illegal to call a member variable as a function while assigning it to itself, because the value of $this->foo is not captured correctly by the closure.