类扩展自我

I stumbled across this code today

class FILTER extends self
{
    protected static function sanitizeHtml($html)
    {
        static $parser;

        if (!isset($parser)) $parser = new HTMLPurifier;

        return $parser->purify($html);
    }
}

What does this class gain from extendingself? Im not quite sure if this ruby example is equivalent or even applicable to PHP.

That's not valid code, wherever you saw it. It throws up an error.

Fatal error: Cannot use 'self' as class name as it is reserved

You can see an example here: https://eval.in/775753