Basic dependency injection container for PHP

ServiceContainer.php

<?php

class ServiceContainer
{
    private static $services;
    private static $generators;

    public function __construct()
    {
        self::$services = [];
        self::$generators = [];
    }

    public function register($name, $generator)
    {
        self::$generators[$name] = $generator;

        return $this;
    }

    public function get($name)
    {
        if (!isset(self::$generators[$name])) {
            throw new ServiceNotDefinedException($name);
        }

        if (!isset(self::$services[$name])) {
            self::$services[$name] = call_user_func(self::$generators[$name], $this);
        }

        $service = self::$services[$name];
        return is_callable($service)
            ? call_user_func($service, $this)
            : $service;
    }

    public function value($value)
    {
        return function ($s) use ($value) {
            return $value;
        };
    }

    public function factory($factory)
    {
        return function ($s) use ($factory) {
            return call_user_func($factory, $s);
        };
    }
}

ServiceNotDefinedException.php

<?php

class ServiceNotDefinedException extends Exception
{
    public function __construct($name)
    {
        $message = sprintf('Service "%s" is not defined', $name);
        parent::__construct($message);
    }
}