Singleton pattern is a creational software design pattern that restricts the instantiation of a class to one “single” instance. This is useful when exactly one object is needed to coordinate actions across the system. It also provides a global endpoint to access the single instance.

For example, Let’s make database connection class to have only one instance across the whole system.

 
<?php declare(strict_types=1);
namespace App\Singleton;
 
class DB {
 
    private static $instance;
    private $connection;
 
    // private constructor
    private final function  __construct() {
        // implement connection to database here
    }
 
    public static function getInstance() {
        if(!isset(self::$instance)) {
            self::$instance = new DB();
        }
        return self::$instance;
    }
 
    public function getConnection() {
        return $this->$connection;
    }
}
?>

Let’s write test cases to verify only single instance is created.

<?php declare(strict_types=1);
namespace App\Singleton\Tests;
 
require_once './vendor/autoload.php'; 
use App\Singleton\DB;
use PHPUnit\Framework\TestCase;
 
class SingletonTest extends TestCase
{
    public function testSingleton()
    {
        $dbObj1 = DB::getInstance();
        $dbObj2 = DB::getInstance();
        $this->assertEquals($dbObj1, $dbObj2);
    }
 
    public function testIsInstanceOfDB()
    {
        $dbObj = DB::getInstance();
        $this->assertInstanceOf(DB::class, $dbObj);
    }
}

Execute tests.

$./vendor/bin/phpunit tests/SingletonTest.php 
PHPUnit 7.5.20 by Sebastian Bergmann and contributors.
 
..                                                                  2 / 2 (100%)
 
Time: 31 ms, Memory: 4.00 MB
 
OK (2 tests, 2 assertions)