class db
{
    /*
    Defines a private variable to be used within the class
    Must be static as we're using it in a static function.
    */
    private static $instance;
    
    /*
    Begin a static singleton function
    These are called like so: db::singleton();
    */
    public static function singleton()
    {
        /*
        Only fill the private variable if it hasn't been
        set on a previous occasion.
        */
        if(!isset(self::$instance))
        {
            /*
            __CLASS__ is a magic constant.
            It is a string that equals "db", in this case
            as that's the name of our current class.
            */
            $c = __CLASS__;
            
            /*
            Assign the value of the new object to our private
            variable.
            */
            self::$instance = new $c();
            // End of lines
        }
    
        /*
        Return the object that references this class.
        */
        return self::$instance;
    }
}

/* To be used like so, function() doesn't need to be static. */
db::singleton()->function();