1.7. Singleton¶
THIS IS CONSIDERED TO BE AN ANTI-PATTERN! FOR BETTER TESTABILITY AND MAINTAINABILITY USE DEPENDENCY INJECTION!
1.7.1. Purpose¶
To have only one instance of this object in the application that will handle all calls.
1.7.2. Examples¶
- DB Connector
- Logger
- Lock file for the application (there is only one in the filesystem …)
1.7.3. UML Diagram¶

1.7.4. Code¶
You can also find this code on GitHub
Singleton.php
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 | <?php
declare(strict_types=1);
namespace DesignPatterns\Creational\Singleton;
use Exception;
final class Singleton
{
private static ?Singleton $instance = null;
/**
* gets the instance via lazy initialization (created on first usage)
*/
public static function getInstance(): Singleton
{
if (static::$instance === null) {
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)
*/
public function __wakeup()
{
throw new Exception("Cannot unserialize singleton");
}
}
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1.7.5. Test¶
Tests/SingletonTest.php
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | <?php
declare(strict_types=1);
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);
}
}
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