Setting dependencies on a container

Setting-PSR11-Container-Dependencies.php

<?php
namespace Kanian\ContainerX;
use Psr\Container\ContainerInterface;

class Container implements ContainerInterface
{
    protected $instances = [];
    //...
    public function set($abstract, $concrete = null)
    {
        if ($concrete === null) {
            $concrete = $abstract;
        }
        $this->instances[$abstract] = $concrete;
    }
  //...   
}