Description:
------------
Currently it's impossible to pass variable by reference to user callback through call_user_func().
There's plenty of complaints about it on the Web.
Such missing possibility would be very useful.

As said in Bug #17309, the situation is not a bug because call_user_func() takes its parameters by value.
Suggestion is to change call_user_func() signature & make it take 2nd & following parameters by reference.

Test script:
---------------
<?php
function foo(&$arg) {
  $arg++;
}
$var = 0;
call_user_func("foo", $var);
echo $var; // echoes 0 instead of 1.
?>

Expected result:
----------------
1

Actual result:
--------------
0