Creating the Singleton design pattern in PHP5

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

    // Prevent cloning and de-serializing
    private function __clone(){}
    private function __wakeup(){}


    /**
     * Call this method to get singleton
     *
     * @return UserFactory
     */
    public static function Instance()
    {
        if ($inst === null) {
            $inst = new UserFactory();
        }
        return $inst;
    }
    
    /**
     * Private ctor so nobody else can instantiate it
     *
     */
    private function __construct()
    {
        
    }
}

To use:

$fact = UserFactory::Instance();
$fact2 = UserFactory::Instance();

$fact == $fact2;

But:

$fact = new UserFactory()

Throws an error.

See http://php.net/manual/en/language.variables.scope.php#language.variables.scope.static to understand static variable scopes and why setting static $inst = null; works.

Unfortunately Inwdr's answer breaks when there are multiple subclasses.

Here is a correct inheritable Singleton base class.

class Singleton
{
    private static $instances = array();
    protected function __construct() {}
    protected function __clone() {}
    public function __wakeup()
    {
        throw new Exception("Cannot unserialize singleton");
    }

    public static function getInstance()
    {
        $cls = get_called_class(); // late-static-bound class name
        if (!isset(self::$instances[$cls])) {
            self::$instances[$cls] = new static;
        }
        return self::$instances[$cls];
    }
}

Test code:

class Foo extends Singleton {}
class Bar extends Singleton {}

echo get_class(Foo::getInstance()) . "\n";
echo get_class(Bar::getInstance()) . "\n";