What is the convention of __ means in php?

I saw some ppl write code like this:

They use __ in the naming of a variable:

   public static function getInstance(){
        if(self::$__instance == NULL) self::$__instance = new SCFormatter();
        return self::$__instance;
    }

not only variable, but also the function:

   private function __clone(){}

Is there any special meaning for php coder to use "__" as a prefix. Thank you.