<?php

namespace Igor\Singleton\Conceptual;

/**
 * The Singleton class defines the `GetInstance` method that serves as an
 * alternative to constructor and lets clients access the same instance of this
 * class over and over.
 */
class Singleton
{
    /**
     * The Singleton's instance is stored in a static field. This field is an
     * array, because we'll allow our Singleton to have subclasses. Each item in
     * this array will be an instance of a specific Singleton's subclass. You'll
     * see how this works in a moment.
     */
    private static $instances = [];

    /**
     * The Singleton's constructor should always be private to prevent direct
     * construction calls with the `new` operator.
     */
    protected function __construct() { }

    /**
     * Singletons should not be cloneable.
     */
    protected function __clone() { }

    /**
     * Singletons should not be restorable from strings.
     */
    public function __wakeup()
    {
        throw new \Exception("Cannot unserialize a singleton.");
    }

    /**
     * This is the static method that controls the access to the singleton
     * instance. On the first run, it creates a singleton object and places it
     * into the static field. On subsequent runs, it returns the client existing
     * object stored in the static field.
     *
     * This implementation lets you subclass the Singleton class while keeping
     * just one instance of each subclass around.
     */
    public static function getInstance(): Singleton
    {
        $cls = static::class;
        if (!isset(self::$instances[$cls])) {
            self::$instances[$cls] = new static();
        }

        return self::$instances[$cls];
    }

    /**
     * Finally, any singleton should define some business logic, which can be
     * executed on its instance.
     */
    public function someBusinessLogic()
    {
        // ...
    }
}

/**
 * The client code.
 */
function clientCode()
{
    $s1 = Singleton::getInstance();
    $s2 = Singleton::getInstance();
    if ($s1 === $s2) {
        echo "Singleton works, both variables contain the same instance.";
    } else {
        echo "Singleton failed, variables contain different instances.";
    }
}

clientCode();
