<?php

class Reference
{
    private $id;

    public function __construct($id) {
        $this->id = strtolower($id);
    }

    public function __toString() {
        return (string) $this->id;
    }
}

class ServiceContainer
{
    private $services;
    private $lastRegistered;

    /**
     * Register a service with the DI container
     *
     * @param string $id Name of the service
     *
     * @param string $class Class to attach to service
     *
     * @return object $this
     */
    public function register($id, $class) {
        $this->addService($id, $class);
        return $this;
    }

    /**
     * Add a parameter to a service
     *
     * @param mixed $parameter Parameter to add
     *
     * @param string $id Name of the service
     *
     * @return object $this
     */
    public function addParameter($parameter, $id = null) {
        $id = isset($id) ? $id : $this->lastRegistered;
        $this->addService($id, null, array($parameter));
        return $this;
    }

    /**
     * Check if a service is registered
     *
     * @param string $id Name of the service
     *
     * @return bool
     */
    public function isRegistered($id) {
        $id = strtolower($id);
        return array_key_exists($id, $this->services);
    }

    /**
     * Add a service to the DI container
     *
     * @param string $id Name of the service
     *
     * @param string $class Class to attach to service
     *
     * @param array $parameters Array of parameters
     */
    public function addService($id, $class, array $parameters = array()) {
        $id = strtolower($id);

        // Initialize the parameter array
        if (! isset($this->services[$id]['parameters'])) $this->services[$id]['parameters'] = array();

        foreach ($parameters as $parameter) {
            $this->services[$id]['parameters'][] = $parameter;
        }

        if (isset($class)) $this->services[$id]['class'] = $class;

        $this->lastRegistered = $id;
    }

    /**
     * Get a service
     *
     * @param string $id Name of the service
     *
     * @return object Return the service
     */
    public function getService($id) {
        $id = strtolower($id);

        if ($this->isRegistered($id)) {
            // Build only when needed
            return (is_object($this->services[$id])) ? $this->services[$id] : $this->buildService($id);
        } else {
            throw new InvalidArgumentException(sprintf('Service "%s" does not exist.', $id));
        }
    }

    /**
     * Build the service
     *
     * This a low level function with all the DI container magic
     *
     * @param string $id Name of the service
     *
     * @return object Return the service
     */
    private function buildService($id) {
        if (! $this->isRegistered($id)) {
            throw new InvalidArgumentException(sprintf('Service "%s" does not exist.', $id));
        }

        if (! isset($this->services[$id]['class'])) {
            throw new RuntimeException(sprintf('Service "%s" has no class to construct.', $id));
        }

        $reflection = new ReflectionClass($this->services[$id]['class']);

        // If the paramters is a reference replace the parameter with the referenced service object
        foreach ($this->services[$id]['parameters'] as &$parameter) {
            if ($parameter instanceof Reference) {
                $parameter = (string) $parameter;
                $parameter = $this->buildService($parameter);
            }
        }

        // Replace the whole array with the service object, this is destructive for the parameters
        //  but they are no longer needed
        // If the constructor is null pass no arguments to the class
        $this->services[$id] = (null === $reflection->getConstructor()) ? $reflection->newInstance() : $reflection->newInstanceArgs($this->services[$id]['parameters']);

        return $this->services[$id];
    }
}

$svc->register('foo', array(5, 'bar'));