<?php

class ClA {

    public $b;

    public function __construct() {
        $this->b = new ClB($this);
    }

    public function x() {
      echo "testing";
    }
}        


class ClB {

    private $a;

    public function __construct($a) {
        $this->a = $a;
    }

    public function c () {
       $this->a->x();
    }
}

$a = new ClA();

$a->b->c();