<?
class class_2{
    public function __construct($callback){
        echo "Hello World - ";
        $missing = $callback();
        $this->missing = $missing;
    }
}
class class_1{
    public function function_1(){
        function callback_function(){
            echo "A Callback. ";
            $missing_variable = "Where Did I Go?";
            return $missing_variable;
        }
        $class2 = new class_2('callback_function');
        return $class2->missing;
    }
    public function __construct(){
        $this->missing = $this->function_1();
    }
}

$class = new class_1();

echo $class->missing;

Hello World - A Callback. Where Did I Go?