PHP singleton mode example sharing, PHP mode example sharing_PHP tutorial

class mysql{
          private static $instance ;//Save instance
//The constructor is declared as private to prevent direct creation of objects
         private function __construct(){
// instance <化>          }
//Single case method, determine whether it has been instantiated, and only instantiate it once
           public static function getInstance (){
If(!isset( self::$instance )){
                      self ::$instance = new self();
            }
               return self:: $instance;
         }
//Prevent cloning objects
           private function __clone (){
              trigger_error ("not allow to clone.");
         }
          function test(){
                  echo "test" ;
         }
}
$conn = mysql::getInstance ();
$conn->test ();
?>