I encountered a weird problem with ReflectionFunction, described in ticket 44139 of PHP Bugs.
If for some reason you need to call with invoke, or invokeArgs, a function like array_unshift (that accepts internally the array by reference) you could use this code to avoid the generated warning or fatal error.
$ref = new ReflectionFunction('array_unshift');
$arguments = func_get_args();
return $ref->invokeArgs(array_merge(array(&$this->arr), $arguments));
I don't know about performances (you can create an array manually too, starting from array(&$this->something) and adding arguments). However, it seems to work correctly without problems, at least until the send by reference will be usable with one single value ...