Cannot pass parameter 2 by reference - php

Question

The code:
final class SimpleEventManager {
private $listeners = array();
public function listen($event, $callable) {
$this->listeners[$event][] = $callable;
}
public function fire($event, array $arguments = array()) {
foreach ($this->listeners[$event] as $listener) {
call_user_func_array($listener, $arguments);
}
}
}
$manager = new SimpleEventManager;
$manager->listen('sql', function($sql) {
$sql .= " order by username desc";
});
$sql = "select * from users";
$manager->fire('sql', array($sql));
var_dump($sql); // is: select * from users
// want: select * from users order by username desc
So basically i want my event listeners to be able to modify the arguments that come in. I've tried doing things like array &$arguments = array() but then I'm getting the Cannot pass parameter 2 by reference error.
Does anyone know how I can solve this?

Answer 1

You can't pass it by reference because only variable may be passed by reference. The literal array($sql) is clearly not a variable.
That said, this isn't the problem.
In fact, there's a lot of problems, mostly because of $sql being "copied" so many times:
When creating the array($sql)
When calling fire() (due to not being passed by reference)
When calling the anonymous function (again, not being passed by reference)
First of all you need to define your array as a variable, such as $arr = array(&$sql);
Then keep your current "fix" of passing &$arguments by reference.
Finally, adjust your anonymous function to function(&$sql) to also work by reference.
All in all, this could be made a lot easier if your code weren't so convoluted ;)