<?php

class foo{
    public $a; //bar

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


    public function scream(){
        echo "scream";
    }
}

class bar {
    public function tryToScream()
    {
        ///?????? Is it a way to call foo->scream() from here ?????
    }
}

$f = new foo(new bar());
$f->a->tryToScream();