Description:
------------
The engine prevents userland code from directly reassigning $this with a compile 
time error, but it does not prevent a number of other mechanisms. The following 
are all possible:

    unset($this);

    // ...

    public function test()
    {
        ${'th'.'is'} = 'foo';
    }

    // ...

    public function test()
    {
        $foo = 'this';
        $$foo = 'foo';
    }

    // ...

    function ref(&$arg)
    {
        $arg = 'foo';
    }

    public function test()
    {
        ref($this);
    }


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

    function ref(&$arg)
    {
        $arg = 'foo';
    }

    class ThisReassignments
    {
        public function test1() { var_dump($this); ${'th'.'is'} = 'foo'; var_dump($this); }
        public function test2() { var_dump($this); $foo = 'this'; $$foo = 'foo';; var_dump($this); }
        public function test3() { var_dump($this); ref($this); var_dump($this); }
    }

    (new ThisReassignments)->test1();
    (new ThisReassignments)->test2();
    (new ThisReassignments)->test3();

     // NB: unset() causes a segmentation fault and doesn't *really* work, but it should emit a meaningful error

Expected result:
----------------
Fatal error with a meaningful error message in all cases

Actual result:
--------------
object(ThisReassignments)#1 (0) {
}
string(3) "foo"
object(ThisReassignments)#1 (0) {
}
string(3) "foo"
object(ThisReassignments)#1 (0) {
}
string(3) "foo"