/**
 * 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()
    {

    }
}

要使用：

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

$fact == $fact2;

但：

$fact = new UserFactory()

引发错误。

请参阅http://php.net/manual/en/language.variables.scope.php#language.variables.scope.static以了解静态变量范围以及设置static $inst = null;工作的原因。