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