Question

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.

Answer 1

__ in PHP is for magic methods like __get, __set, __clone

Historically _ before variable or function means it’s private since there was no private, public or protected methods in PHP 4.

It is applied not only to PHP. In Python for example, _ prefix for functions and variables is used for the same purpose.

I suggest to avoid naming your functions prefixing them with __ (double underscore) since PHP developers can add the same magic function in next versions of the language, so it would break your code. You can use one underscore still – it is not dangerous, since it can’t affect the language features.
Although it’s possible, please notice that using _ for property / method name prefix is not advised since it doesn’t comply with most of the coding standards nowadays.