Function.php

<?php

class A {

    private $e;

    public function __construct(){
        $this->e = new B();
    }

    public function __call(string $name, array $arguments) {
        if (!method_exists($this, $name)) {
            if (method_exists($this->e, $name)) {
                return $this->e->$name($arguments);
            }
            throw new \Error("Call to undefined function \"{$name}()\" (#" . count($arguments) . ")");
        } else {
            return $this->$name($arguments);
        }
    }

    public function e() {
        return __CLASS__ . " : E";
    }

}

class B {

    public function f() {
        return __CLASS__ . " : F";
    }

}

$a = new A();

echo $a->f(); //prints 'B : F'
echo $a->z(); //throws error