Creating the Singleton design pattern in PHP5

Question

How would one create a Singleton class using PHP5 classes?

Answer 1

I agree with the first answer but I would also declare the class as final so that it cannot be extended as extending a singleton violates the singleton pattern. Also the instance variable should be private so that it cannot be accessed directly. Also make the __clone method private so that you cannot clone the singleton object.

Below is some example code.

/**
 * Singleton class
 *
 */
final class UserFactory
{
    private static $_instance = null;

    /**
     * Private constructor
     *
     */
    private function __construct() {}

    /**
     * Private clone method
     *
     */
     private function __clone() {}

    /**
     * Call this method to get singleton
     *
     * @return UserFactory
     */
    public static function getInstance()
    {
        if (self::$_instance === null) {
            self::$_instance = new UserFactory();
        }
        return self::$_instance;
    }
}

Example Usage

$user_factory = UserFactory::getInstance();

What this stops you from doing (which would violate the singleton pattern..

YOU CANNOT DO THIS!

$user_factory = UserFactory::$_instance;

class SecondUserFactory extends UserFactory { }