I have a function that requires as arguments a reference to an array and a reference to a scalar variable. This function passes those arguments to another function, which supplies them to a callback function when it gets executed at a later time, e.g., via call_user_func_args. In some cases, the callback may not require an array of arguments or return a value. Therefore, I want to provide an empty array and a null scalar as default arguments. My question is, is it legal to use a variable's name as a default argument in the function's declaration, as shown in this code?

PHP Code:
  $empty_array = array();
$null_output null;

add_idle($callback,&$args $empty_array,&$ret $null_output) {
   try {
$handle rand_str(16);
$idle IdleTaskList::getInstance();
   catch (
Exception $e) {
// add_idle