wynik po właściwości obiektu

<?php

Class Car {
	private $id;
	
	public function __construct($id){
		$this->id = $id;
	}
	
	public function getId(){
		return $this->id;
	}
}

$cars = [ new Car(3), new Car(4)];


$res = array_combine(
	array_map(function ($o) { 
		return $o->getId(); 
	}, $cars), $cars
);

//
array(2) {
  [3]=>
  object(Car)#1 (1) {
    ["id":"Car":private]=>
    int(3)
  }
  [4]=>
  object(Car)#2 (1) {
    ["id":"Car":private]=>
    int(4)
  }
}