&$this in callback seems to work without complains, as you said.

The following does not work
```php
function f($v) { return $v++; }
$a = 1;
$b = f(&$a);
var_dump($a, $b);
```
`PHP Fatal error:  Call-time pass-by-reference has been removed; If you would like to pass argument by reference, modify the declaration of f().`