Same class, separate instances, can change sibling object's private variables. What the hell, PHP?

Nonsense.php

<?php

ini_set('display_errors', true);

class Car
{
	private $make;

	public function __construct($make)
	{
		$this->make = $make;
	}

	public function __toString()
	{
		return $this->make;
	}
	
	public function eat(Car $c)
	{
		$c->make = 'Crushed '.$c->make;
	}
}


$weaksauce = new Car('Nissan Juke');
$truck = new Car('Monster Truck');


echo 'Weaksauce: '.$weaksauce."\n";

$truck->eat($weaksauce);

echo 'Weaksauce: '.$weaksauce."\n";

hmm, yeah that's weird. I didn't know that was possible .. it shoulnd't be.