截取GBK字符串的有关问题~

function  gb_substr( $str ,  $len ){        
      $count  = 0;        
      for ( $i =0;  $i < strlen ( $str );  $i ++){        
          if ( $count  ==  $len )  break ;        
          if (preg_match( "/[\x80-\xff]/" ,  substr ( $str ,  $i , 1))){
        ++ $i ;
          }        
          ++ $count ;                
      }        
      return   substr ( $str , 0,  $i );        
}