petrknap/php-singleton

Singleton pattern for PHP

v1.0.0 2016-09-22 07:59 UTC

README

What is Singleton pattern?

In software engineering, the singleton pattern is a design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized to systems that operate more efficiently when only one object exists, or that restrict the instantiation to a certain number of objects. The term comes from the mathematical concept of a singleton. -- Singleton pattern - Wikipedia, The Free Encyclopedia

Why use Singletons?

Because it is deadlock-safe. Don't trust me? Let see at this code:

<?php

class MyFileAppender
{
    const MY_FILE = "./my.file";

    private $handle = null;

    public function __construct()
    {
        $this->handle = fopen(self::MY_FILE, 'a');
        flock($this->handle, LOCK_EX);
    }

    public function __destruct()
    {
        flock($this->handle, LOCK_UN);
        fclose($this->handle);
    }
}

$first = new MyFileAppender();  // OK
$second = new MyFileAppender(); // Deadlock

And now the same code with Singleton pattern:

<?php

class MyFileAppender implements \PetrKnap\Php\Singleton\SingletonInterface
{
    use \PetrKnap\Php\Singleton\SingletonTrait;

    const MY_FILE = "./my.file";

    private $handle = null;

    private function __construct()
    {
        $this->handle = fopen(self::MY_FILE, 'a');
        flock($this->handle, LOCK_EX);
    }

    public function __destruct()
    {
        flock($this->handle, LOCK_UN);
        fclose($this->handle);
    }
}

$first = MyFileAppender::getInstance();  // OK
$second = MyFileAppender::getInstance(); // OK

Usage of php-singleton

Singleton declaration

<?php

class MySingleton implements \PetrKnap\Php\Singleton\SingletonInterface
{
    use \PetrKnap\Php\Singleton\SingletonTrait;
    
    private $name;
    
    private function __constructor() // WARNING: Don't use public constructor - it's singleton!
    {
        // If you need some construct logic place it here, otherwise don't declare this method
    }
    
    public function getName()
    {
        return $this->name;
    }
    
    public function setName($name)
    {
        $this->name = $name;
    }
}

Singleton usage

<?php

$a = MySingleton::getInstance();
$b = MySingleton::getInstance();

if ($a == $b) {
    echo "Both variables contain the same instance.";
}

$a->setName("Singleton A");

if ($b->getName() == "Singleton A") {
    echo "Both variables contain the same instance.";
}

How to install

Run composer require petrknap/php-singleton or merge this JSON code with your project composer.json file manually and run composer install. Instead of dev-master you can use one of released versions.

{
    "require": {
        "petrknap/php-singleton": "dev-master"
    }
}

Or manually clone this repository via git clone https://github.com/petrknap/php-singleton.git or download this repository as ZIP and extract files into your project.