class DI
{
    private static $container = [];

    public static function set($key, $closure)
    {
        self::$container[$key] = $closure;
    }

    public static function get($key)
    {
        return self::$container[$key]();
    }
}

DI::set('request', function(){
    return new Kernel\Request();
});

$request = DI::get('request');

Fatal error: Function name must be a string in C:\OpenServer\domains\mvc.loc\protected\classes\Kernel\DI.php on line 15

public static function get($key)
    {
        return self::$container[$key]->__invoke();
    }

public static function get($key)
{
    $closure = self::$container[$key];

    return $closure();
}

$closure = self::$container[$key];
return $closure();

return self::$container[$key]();

$a = 5;
echo $a;

echo $a = 5;