public function getReadableFileSize($retstring=null){

        $sizes = array('byte','kB','MB','GB','TB','PB','EB','ZB','YB');

        if($retstring == null)

           $retstring = "%01.2f %s";

       $lastsizestring = end($sizes);

       foreach($sizes as $sizestring){

           if($this->size<1024)

                 break;

           if($sizestring!=$lastsizestring)

              $this->size/=1024;

      }                        

       if($sizestring == $sizes[0])

           $retstring = "%01d %s":

       return sprintf($retstring,$this->size,$sizestring);

 }