smallest_possible_dependency_injection_container.php

<?php

class Container 
{
    public $bindings = [];

    /**
     * @param $key
     * @param $value
     */
    public function __set($key, $value)
    {
        $this->bindings[$key] = $value;
    }

    /**
     * @param $key
     * @return mixed
     */
    public function __get($key)
    {
        return $this->bindings[$key]();
    }
}
class Car 
{
    public function __construct()
    {
        echo get_class() . "\n";
    }
}
class Truck 
{
    public function __construct()
    {
        echo get_class() . "\n";
    }
}

$container = new Container;
$container->car = function(){ new Car;};
$container->truck = function(){ new Truck;};
$container->truck;