function _duration_string( $duration ) {
   $duration = (string) trim( $duration );
   while( 'P' != strtoupper( substr( $duration, 0, 1 ))) {
     if( 0 < strlen( $duration ))
       $duration = substr( $duration, 1 );
     else
       return false; // no leading P !?!?
   }
   $duration = substr( $duration, 1 ); // skip P
   $duration = str_replace ( 't', 'T', $duration );
   $duration = str_replace ( 'T', '', $duration );
   $output = array();
   $val    = null;
   for( $ix=0; $ix < strlen( $duration ); $ix++ ) {
     switch( strtoupper( $duration{$ix} )) {
      case 'W':
        $output['week'] = $val;
        $val            = null;
        break;
      case 'D':
        $output['day']  = $val;
        $val            = null;
        break;
      case 'H':
        $output['hour'] = $val;
        $val            = null;
        break;
      case 'M':
        $output['min']  = $val;
        $val            = null;
        break;
      case 'S':
        $output['sec']  = $val;
        $val            = null;
        break;
      default:
        if( !ctype_digit( $duration{$ix} ))
          return false; // unknown duration controll character  !?!?
        else
          $val .= $duration{$ix};
     }
   }
   return $this->_duration_array( $output );
  }