ServiceLocator.php

<?php

namespace DesignPatterns\More\ServiceLocator;

class ServiceLocator implements ServiceLocatorInterface
{
    /**
     * All services.
     *
     * @var array
     */
    private $services;

    /**
     * The services which have an instance.
     *
     * @var array
     */
    private $instantiated;

    /**
     * True if a service can be shared.
     *
     * @var array
     */
    private $shared;

    public function __construct()
    {
        $this->services     = array();
        $this->instantiated = array();
        $this->shared       = array();
    }

    /**
     * Registers a service with specific interface.
     *
     * @param string $interface
     * @param string|object $service
     * @param bool $share
     */
    public function add($interface, $service, $share = true)
    {
        /**
         * When you add a service, you should register it
         * with its interface or with a string that you can use
         * in the future even if you will change the service implementation.
         */

        if (is_object($service) && $share) {
            $this->instantiated[$interface] = $service;
        }
        $this->services[$interface] = (is_object($service) ? get_class($service) : $service);
        $this->shared[$interface]   = $share;
    }

    /**
     * Checks if a service is registered.
     *
     * @param string $interface
     *
     * @return bool
     */
    public function has($interface)
    {
        return (isset($this->services[$interface]) || isset($this->instantiated[$interface]));
    }

    /**
     * Gets the service registered for the interface.
     *
     * @param string $interface
     *
     * @return mixed
     */
    public function get($interface)
    {
        // Retrieves the instance if it exists and it is shared
        if (isset($this->instantiated[$interface]) && $this->shared[$interface]) {
            return $this->instantiated[$interface];
        }

        // otherwise gets the service registered.
        $service = $this->services[$interface];

        // You should check if the service class exists and
        // the class is instantiable.

        // This example is a simple implementation, but
        // when you create a service, you can decide
        // if $service is a factory or a class.
        // By registering a factory you can create your services
        // using the DependencyInjection pattern.

        // ...

        // Creates the service object
        $object = new $service();

        // and saves it if the service must be shared.
        if ($this->shared[$interface]) {
            $this->instantiated[$interface] = $object;
        }
        return $object;
    }
}