App.php
<?php

namespace core;

class App {

    protected static $container;

    public static function setContainer($container){

        static::$container = $container;

    }

    public static function Container(){

        return static::$container;

    }

    public static function resolve($key){

       return App::$container->resolve($key);

    }

}

Container.php
<?php

namespace core;

class Container{

    protected $bindings=[];

    public function bind($key, $resolver){

        $this->bindings[$key] = $resolver;

    }

    public function resolve($key){

        if (!array_key_exists($key, $this->bindings)){

            throw new \Exception('No matching binding found for ' . $key);

        }

        $resolver = $this->bindings[$key];
        return call_user_func($resolver);

    }

}

Bootstrap.php
<?php
use core\App;
use core\Container;
use core\Database;
use core\Session;

$container = new Container();

$container->bind('core\Database', function (){

$config = require base_path("src/config.php");
return new Database($config['database'], $config['database']['user'], $config['database']['password']);

});

$container->bind('core\Session', function(){
    return new Session;
});

App::setContainer($container);