动态转换GB->UTF-8编码至少有四种方法：

用PHP的iconv扩展转换

用PHP的mb_string扩展转换

用对换表转换，对换表存储在MySQL数据库中

用对换表转换，对换表存储在文本文件中

　　前两种方法要服务器作了相应设置（编译安装了相应扩展）才能使用。我的虚拟主机没有这两个扩展，只好考虑后两种方法。前两个方法本文也不进行测评。

　　测评程序如下（func_ip.php参见《IP地址->地理位置转换的测评》一文）：

<?php

require_once ("func_ip.php");

function u2utf8($c) {

  $str = "";

  if ($c < 0x80) {

    $str .= $c;

  } elseif ($c < 0x800) {

    $str .= chr(0xC0 | $c >> 6);

    $str .= chr(0x80 | $c & 0x3F);

  } elseif ($c < 0x10000) {

    $str .= chr(0xE0 | $c >> 12);

    $str .= chr(0x80 | $c >> 6 & 0x3F);

    $str .= chr(0x80 | $c & 0x3F);

  } elseif ($c < 0x200000) {

    $str .= chr(0xF0 | $c >> 18);

    $str .= chr(0x80 | $c >> 12 & 0x3F);

    $str .= chr(0x80 | $c >> 6 & 0x3F);

    $str .= chr(0x80 | $c & 0x3F);

  }

  return $str;

}

function GB2UTF8_SQL($strGB) {

  if (!trim($strGB)) return $strGB;

  $strRet = "";

  $intLen = strlen($strGB);

  for ($i = 0; $i < $intLen; $i++) {

    if (ord($strGB{$i}) > 127) {

        $strCurr = substr($strGB, $i, 2);

        $intGB = hexdec(bin2hex($strCurr)) - 0x8080;

        $strSql = "SELECT code_unicode FROM nnstats_gb_unicode

          WHERE code_gb = ".$intGB." LIMIT 1"

        ;

        $resResult = mysql_query($strSql);

        if ($arrCode = mysql_fetch_array($resResult)) $strRet .= u2utf8($arrCode["code_unicode"]);

        else $strRet .= "??";

        $i++;

    } else {

        $strRet .= $strGB{$i};

    }

  }

  return $strRet;

}

function GB2UTF8_FILE($strGB) {

  if (!trim($strGB)) return $strGB;

  $arrLines = file("gb_unicode.txt");

  foreach ($arrLines as $strLine) {

    $arrCodeTable[hexdec(substr($strLine, 0, 6))] = hexdec(substr($strLine, 7, 6));

  }

  $strRet = "";

  $intLen = strlen($strGB);

  for ($i = 0; $i < $intLen; $i++) {

    if (ord($strGB{$i}) > 127) {

        $strCurr = substr($strGB, $i, 2);

        $intGB = hexdec(bin2hex($strCurr)) - 0x8080;

        if ($arrCodeTable[$intGB]) $strRet .= u2utf8($arrCodeTable[$intGB]);

        else $strRet .= "??";

        $i++;

    } else {

        $strRet .= $strGB{$i};

    }

  }

  return $strRet;

}

function EncodeIp($strDotquadIp) {

  $arrIpSep = explode('.', $strDotquadIp);

  if (count($arrIpSep) != 4) return 0;

  $intIp = 0;  

  foreach ($arrIpSep as $k => $v) $intIp += (int)$v * pow(256, 3 - $k);

  return $intIp;

  //return sprintf('%02x%02x%02x%02x', $arrIpSep[0], $arrIpSep[1], $arrIpSep[2], $arrIpSep[3]);

}

function GetMicroTime() {

  list($msec, $sec) = explode(" ", microtime());

  return ((double)$msec + (double)$sec);

}

for ($i = 0; $i < 100; $i++) { // 随机产生100个ip地址

  $strIp = mt_rand(0, 255).".".mt_rand(0, 255).".".mt_rand(0, 255).".".mt_rand(0, 255);

  $arrAddr[$i] = ip2addr(EncodeIp($strIp));

}

$resConn = mysql_connect("localhost", "netnest", "netnest");

mysql_select_db("test");

// 测评MySQL查询的编码转换

$dblTimeStart = GetMicroTime();

for ($i = 0; $i < 100; $i++) {

  $strUTF8Region = GB2UTF8_SQL($arrAddr[$i]["region"]);

  $strUTF8Address = GB2UTF8_SQL($arrAddr[$i]["address"]);

}

$dblTimeDuration = GetMicroTime() - $dblTimeStart;

// 测评结束并输出结果

echo $dblTimeDuration; echo " ";

// 测评文本文件查询的编码转换

$dblTimeStart = GetMicroTime();

for ($i = 0; $i < 100; $i++) {

  $strUTF8Region = GB2UTF8_FILE($arrAddr[$i]["region"]);

  $strUTF8Address = GB2UTF8_FILE($arrAddr[$i]["address"]);

}

$dblTimeDuration = GetMicroTime() - $dblTimeStart;

// 测评结束并输出结果

echo $dblTimeDuration; echo " ";

?>

测评两次结果（精确到3位小数，单位是秒）：

MySQL