There is no reference sign on a function call - only on function definitions

Function definitions alone are enough to correctly pass the
argument by reference. As of PHP 5.3.0, you will get a warning saying
that "call-time pass-by-reference" is deprecated when you use & in
foo(&$a);. And as of PHP 5.4.0, call-time pass-by-reference was removed,
so using it will raise a fatal error.