Little typo, i meant:

set_error_handler('errorHandler');

I just realized that a similar effect can be reached by setting the reference mark before the parameter name. The problem is just that the original object will be changed too.

function a(&$a){
      var_dump($a);
}

$test = new A();
var_dump($test); // object A
a($test) // int 1
var_dump($test); // int 1 => this should still be object A, you should create internally a new reference parameter variable referencing the passed object if you know what i mean.

I know that native values get copied and that there is no problem with overwriting the value. But objects could be handled like this: if used as parameter value, create a new reference variable referencing the object, this variable can then be overwritten without changing the original object.

I can't reproduce this. https://3v4l.org/QI9H8 shows that across all PHP versions it's not possible to modify the argument via debug_backtrace(). That's the correct behavior -- you aren't supposed to be able to do a modification through that. If you are able to do changes, that would be a bug.

I think I have what @blue-tidus159 was trying to show. The key is that the function parameter has a type hint and the value passed is not an instance of it (thus triggering the error). But it gets a lot more interesting after that...
https://3v4l.org/7B3jB

Then the comment later seems to be about how it also happens with by-ref parameters.
https://3v4l.org/Vs8lG

On a lighter note, I have been inspired to create this abomination:
https://3v4l.org/5lbf0

@requinix: Thanks, I didn't get that this is about type hints. In that case, I guess this is fixed in PHP 7 by means of it throwing an exception? It doesn't seem like doing this kind of modification through a direct call to debug_backtrace() is possible: https://3v4l.org/MWE8l

As per previous comment, no longer relevant in PHP 7.