<?php


class A
{
public function myFunction(){}
}


////
class B
{
private $a;
public function __construct($obj){ $this->a = $obj);
public function callMyFunction(){$this->a->myFunction();}
}

$b = new B(new A());

$b->callMyFunction();


or

////

class B
{
public callMyFunction($b) { $b->myFunction(); }
}

$b = new B();
$b->callMyFunction(new A());