1.8. Singleton

THIS IS CONSIDERED TO BE AN ANTI-PATTERN! FOR BETTER TESTABILITY ANDMAINTAINABILITY USE DEPENDENCY INJECTION!

1.8.1. Purpose

To have only one instance of this object in the application that willhandle all calls.

1.8.2. Examples

• DB Connector
• Logger (may also be a Multiton if there are many log files forseveral purposes)
• Lock file for the application (there is only one in the filesystem…)

1.8.3. UML Diagram

1.8.4. Code

You can also find this code on GitHub

Singleton.php

<?php
 
namespace DesignPatterns\Creational\Singleton;
 
final class Singleton
{
    /**
     * @var Singleton
     */
    private static $instance;
 
    /**
     * gets the instance via lazy initialization (created on first usage)
     */
    public static function getInstance(): Singleton
    {
        if (null === static::$instance) {
            static::$instance = new static();
        }
 
        return static::$instance;
    }
 
    /**
     * is not allowed to call from outside to prevent from creating multiple instances,
     * to use the singleton, you have to obtain the instance from Singleton::getInstance() instead
     */
    private function __construct()
    {
    }
 
    /**
     * prevent the instance from being cloned (which would create a second instance of it)
     */
    private function __clone()
    {
    }
 
    /**
     * prevent from being unserialized (which would create a second instance of it)
     */
    private function __wakeup()
    {
    }
}

1.8.5. Test

Tests/SingletonTest.php

<?php
 
namespace DesignPatterns\Creational\Singleton\Tests;
 
use DesignPatterns\Creational\Singleton\Singleton;
use PHPUnit\Framework\TestCase;
 
class SingletonTest extends TestCase
{
    public function testUniqueness()
    {
        $firstCall = Singleton::getInstance();
        $secondCall = Singleton::getInstance();
 
        $this->assertInstanceOf(Singleton::class, $firstCall);
        $this->assertSame($firstCall, $secondCall);
    }
}