PHP call_user_function using references

Here's main application:

$test=array();
invoke_hooks("hook",$test);

Two functions of hook:

function first_hook(&$a = array()) {
    $a[40] = "1";
}

function second_hook(&$a = array()) {
    $a[300] = "2";
}

function invoke_hooks($hook_name) {
    $args = func_get_args();
    array_shift($args);
    $functions = get_defined_functions();
    foreach ($functions as $f) {
        if (substr($f, -strlen($hook_name)) === $hook_name) {
            call_user_func_array($f, &$args);
        }
    }
}

The idea is to get :

$test[40]=="1";
$test[300]=="2";

How can I make it?