<?php

class Foo {
    public $bar;

    public function setBar($bar)
    {
        $this->bar = $bar;
    }
}

class Init {

    public function initFoo(Foo $foo)
    {
        $foo->setBar(5);
        //return $foo???
    }
}

$foo = new Foo();
$init = new Init();
$init->initFoo($foo);