PHP Design Patterns: Singleton

What is Singleton?

Singleton is a creational design pattern, which ensures that only one object of this type exists and provides a single point of access to it for any other code.

Just like a global variable, the Singleton pattern allows you to access an object from anywhere in the program. However, it also protects that instance from being overwritten by other code.

Example

Step 1 - Directory System:

📦 Singleton
 ┣ 📜 Preferences.php
 ┣ 📜 Application.ini
 ┗ 📜 index.php

Step 2 - Preferences Class:

<?php

namespace Singleton;

class Preferences
{
    private array $data;
    private static object $instance;
}

• construct method

// Constructor method is private so we can have a global object.
private function __construct()
{
    // The data property receives information from the application file.
    $this->data = parse_ini_file('application.ini');
}

• getInstance method

// Method for creating an instance or rescuing an already instantiated object.
public static function getInstance(): object
{
    // if there is no instance.
    if (empty(self::$instance)) {
        // Create a new instance.
        self::$instance = new self();
    }

    // return instance.
    return self::$instance;
}

• setData and getData methods

// Method for getting information.
public function getData(string $key): mixed
{
    return $this->data[$key];
}

// Method for inserting or changing information.
public function setData(string $key, mixed $value): void
{
    $this->data[$key] = $value;
}

• save method

// Method for writing information to the file application.ini
public function save(): void
{
    // Variable for information.
    $string = '';
    // If there is any information on date.
    if ($this->data) {
        // Traverses the data vector.
        foreach ($this->data as $key => $value) {
            // Fills the string variable with the information.
            $string .= "{$key} = \"{$value}\" \n";
        }
    }

    // Write information to the file.
    file_put_contents('application.ini', $string);
}

Testing

<?php

require_once 'Preferences.php';

use Singleton\Preferences;

$p1 = Preferences::getInstance();
var_dump($p1);

$p2 = Preferences::getInstance();
var_dump($p2);

object(Singleton\Preferences)[1]
  private array 'data' => 
    array (size=0)
      empty

object(Singleton\Preferences)[1]
  private array 'data' => 
    array (size=0)
      empty

Insert Data

<?php

require_once 'Preferences.php';

use Singleton\Preferences;

$p1 = Preferences::getInstance();
$p1->setData('Language', 'en');
$p1->save();

$p2 = Preferences::getInstance();
print $p2->getData('Language');
var_dump($p2);

Language: en

object(Singleton\Preferences)[1]
  private array 'data' => 
    array (size=1)
      'Language' => string 'en' (length=2)

Objects p1 and p2 have the same instance.

Comments

[websilvercraft]

Singleton pattern makes the code less flexible, because you can not extend the singleton class, so it breaks 2 of the solid design principles: open close principle and and single responsibility principle. I find the alternative implementations of singleton pattern in php, using functions, more flexible.