function ZHsubstr($str, $start=0, $length, $charset="utf-8"){
      $len = strlen($str); //原字节数
     if($len > $length)
            $suffixStr = "…";
     else
             $suffixStr = "";

   if(function_exists("mb_substr"))
          return mb_substr($str, $start, $length, $charset).$suffixStr;
   elseif(function_exists('iconv_substr')) {
        return iconv_substr($str,$start,$length,$charset).$suffixStr;
    }
    $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 $slice.$suffixStr;
}

使用方法：
 
$str = "$page_title";  
$page_title=ZHsubstr($str, 0, 40, 'utf-8');