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Singleton Design Pattern in PHP: How to Use It Correctly

Chris06 mins
Singleton Design Pattern

The Singleton Design Pattern in PHP is one of the most misunderstood patterns in software development. While it’s commonly used to restrict object creation to a single instance, poor implementation can lead to tightly coupled code and testing nightmares. In this guide, we’ll show how to properly implement the singleton pattern, its use cases, and common mistakes to avoid.

What Is the Singleton Design Pattern in PHP?

This design pattern in PHP ensures that a class has only one instance and provides a global access point to it. This is particularly useful in cases like database connections or logging systems, where having multiple instances can lead to unnecessary resource usage or inconsistent state.

A singleton is not just about limiting instantiation. It’s about controlled access to a unique resource. If used correctly, it can improve performance and memory efficiency. But if used improperly, it can break object-oriented principles such as loose coupling and testability.

Let’s start by looking at a bad singleton implementation, so we can understand what not to do.

Bad Example of Singleton Design Pattern in PHP

class BadLogger {
    private static $instance;

    private function __construct() {
        // open file or set up resource
    }

    public static function getInstance() {
        if (self::$instance === null) {
            self::$instance = new BadLogger();
        }
        return self::$instance;
    }

    public function log($message) {
        echo "Logging: $message";
    }
}

// Usage
$log = BadLogger::getInstance();
$log->log("Something happened");

What’s wrong here?

  • No prevention of cloning or unserializing: Anyone can bypass the singleton using clone or unserialize.
  • Tightly coupled to global state: Makes unit testing harder.
  • No interface or abstraction: Not extendable or replaceable.

Good Example of Singleton Design Pattern in PHP

final class Logger {
    private static ?Logger $instance = null;

    private function __construct() {
        // Set up the logging mechanism
    }

    private function __clone() {}
    private function __wakeup() {}

    public static function getInstance(): Logger {
        if (self::$instance === null) {
            self::$instance = new self();
        }
        return self::$instance;
    }

    public function log(string $message): void {
        echo "[LOG]: " . $message . PHP_EOL;
    }
}

// Usage
$logger = Logger::getInstance();
$logger->log("System started");

Why this is better:

  • Constructor is private: Prevents direct instantiation.
  • Clone and unserialize are disabled: Enforces true singleton behavior.
  • Final class: Prevents subclassing and maintaining multiple states.
  • Typed properties and return types: Better readability and safer code.

When to Use it

Use the Singleton Design Pattern in PHP when:

  • You need to manage a shared resource like a database connection, configuration settings, or logging service.
  • Creating multiple instances would be costly or error-prone.
  • Global state is necessary and can’t be easily injected.

However, avoid using singletons in:

  • Business logic layers.
  • Scenarios requiring unit testing or mocking.
  • Codebases aiming for dependency injection and loose coupling.

Making It Test-Friendly

One of the biggest criticisms of the Singleton Design Pattern in PHP is that it’s hard to test. Since the instance is static and globally accessible, mocking or resetting the state becomes a challenge.

Pro tip: Use a wrapper or dependency injection container.

Instead of directly using the singleton inside your classes, inject it:

class UserService {
    private Logger $logger;

    public function __construct(Logger $logger) {
        $this->logger = $logger;
    }

    public function createUser($name) {
        // create user logic
        $this->logger->log("Created user: $name");
    }
}

Now you can inject a mock Logger in your tests and still retain the benefits of singleton during runtime.

Conclusion: Use Singleton Pattern Responsibly

The singleton approach has both advantages and limitations. Use it when you need a controlled, single access point to a shared resource. But don’t let it become a crutch. Poorly managed singletons can lead to tightly coupled, hard-to-test codebases.

By applying best practices—like sealing the class, disabling clone/unserialize, and injecting the instance—you get all the benefits without sacrificing maintainability.

Interested in More Design Patterns?

Discover other powerful patterns like Factory, Strategy, and Observer in this full guide:

➡️ Design Patterns in PHP – The Complete Guide to All Types

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