字符串截取，支持中文和其他编码--来自thinkPhp

/**
 * 字符串截取，支持中文和其他编码
 *
 * @access public
 * @param string $str
 *         需要转换的字符串
 * @param string $start
 *         开始位置
 * @param string $length
 *         截取长度
 * @param string $charset
 *         编码格式
 * @param string $suffix
 *         截断显示字符
 * @return string
 */
function msubstr($str, $start = 0, $length, $charset = "utf-8", $suffix = true) {
 if (function_exists ( "mb_substr" ))
  $slice = mb_substr ( $str, $start, $length, $charset );
 elseif (function_exists ( 'iconv_substr' )) {
  $slice = iconv_substr ( $str, $start, $length, $charset );
  if (false === $slice) {
   $slice = '';
  }
 } else {
  $re ['utf-8'] = "/[\x01-\x7f]|[\xc2-\xdf][\x80-\xbf]|[\xe0-\xef][\x80-\xbf]{2}|[\xf0-\xff][\x80-\xbf]{3}/";
  $re ['gb2312'] = "/[\x01-\x7f]|[\xb0-\xf7][\xa0-\xfe]/";
  $re ['gbk'] = "/[\x01-\x7f]|[\x81-\xfe][\x40-\xfe]/";
  $re ['big5'] = "/[\x01-\x7f]|[\x81-\xfe]([\x40-\x7e]|\xa1-\xfe])/";
  preg_match_all ( $re [$charset], $str, $match );
  $slice = join ( "", array_slice ( $match [0], $start, $length ) );
 }
 return $suffix ? $slice . '...' : $slice;
}