class A {
    protected function __construct(){
    // made protected to disallow calling with $aa = new A()
    // in fact, does nothing
    };

    static public function create(){
        $instance = new self();
        //... some important code
        return $instance;
    }

    static public function createFromYetAnotherClass(YetAnotherClass $xx){
        // ...
    }

class B extends A {};

$aa = A::create();
$bb = B::create();