There are several other ways to also re-assign $this, e.g.:

class Test {
 public function __construct() {
  var_dump($this);

  /* Quirk comes in here by using the concation operator */
  /* ${'this'} is not possible and results in an E_ERROR */

  ${'t' . 'his'} = 'Hello';
  var_dump($this);
 }
}

If any whether this should be fixed within the Engine, then Dmitry would know so I'm re-assigning it to him for him to decide. As for a documentation issue, I really don't belive we should document such quirks in the official documentation else people will rely on such buggy "features".

It's possible to do it even without dynamic tricks.

<?php
class Foo { 
  function bar() { 
    $x = &$this;
    $x = null;
    var_dump($this);
  }
} 
$x = new Foo();
$x->bar();
?>

I would say I don't like to fix all these issues, because the fixes would slowdown absolutely legal PHP code.