下面的例子是将 gb2312 转换为 "全"这种形式

php4.3.1以后的iconv函数很好用的，只是需要自己写一个uft8到unicode的转换函数
查表(gb2312.txt)也行
<?
 $text = "电子书库";
preg_match_all("/[x80-xff]?./", $text, $ar);
foreach( $ar[0] as  $v)
echo "&#".utf8_unicode(iconv("GB2312","UTF-8", $v)).";";
?>
<?
// utf8 -> unicode
function utf8_unicode( $c) {
switch(strlen( $c)) {
case 1:
return ord( $c);
case 2:
 $n = (ord( $c[0]) & 0x3f) << 6;
 $n = ord( $c[1]) & 0x3f;
return  $n;
case 3:
 $n = (ord( $c[0]) & 0x1f) << 12;
 $n = (ord( $c[1]) & 0x3f) << 6;
 $n = ord( $c[2]) & 0x3f;
return  $n;
case 4:
 $n = (ord( $c[0]) & 0x0f) << 18;
 $n = (ord( $c[1]) & 0x3f) << 12;
 $n = (ord( $c[2]) & 0x3f) << 6;
 $n = ord( $c[3]) & 0x3f;
return  $n;
}
}
?>

下面的例子是利用php将"&#20840;"这中编码转换为gb2312.
<?php
 $str = "TTL&#20840;&#22825;&#20505;&#33258;&#21160;&#32858;&#28966;";
 $str = preg_replace("|&#([0-9]{1,5});|", "".u2utf82gb(\1)."",  $str);
 $str = " $str=" $str";";

eval( $str);
echo  $str;

function u2utf82gb( $c){
 $str="";
if ( $c < 0x80) {
 $str.= $c;
} else if ( $c < 0x800) {
 $str.=chr(0xC0 |  $c>>6);
 $str.=chr(0x80 |  $c & 0x3F);
} else if ( $c < 0x10000) {
 $str.=chr(0xE0 |  $c>>12);
 $str.=chr(0x80 |  $c>>6 & 0x3F);
 $str.=chr(0x80 |  $c & 0x3F);
} else if ( $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 iconv('UTF-8', 'GB2312',  $str);
}
?>

或者是

function unescape( $str) {
 $str = rawurldecode( $str);
preg_match_all("/(?:%u.{4})|&#x.{4};|&#d ;|. /U", $str, $r);
 $ar =  $r[0];
print_r( $ar);
foreach( $ar as  $k=> $v) {
if(substr( $v,0,2) == "%u")
 $ar[ $k] = iconv("UCS-2","GB2312",pack("H4",substr( $v,-4)));
elseif(substr( $v,0,3) == "&#x")
 $ar[ $k] = iconv("UCS-2","GB2312",pack("H4",substr( $v,3,-1)));
elseif(substr( $v,0,2) == "&#") {
echo substr( $v,2,-1)."<br>";
 $ar[ $k] = iconv("UCS-2","GB2312",pack("n",substr( $v,2,-1)));
}
}
return join("", $ar);
}

 $str = "TTL&#20840;&#22825;&#20505;&#33258;&#21160;&#32858;&#28966;";
echo unescape( $str); //out TTL全天候自动聚焦

利用javascript来转换

<style>
BODY {
FONT-SIZE: 9pt; PADDING-RIGHT: 0px; PADDING-LEFT: 0px; PADDING-BOTTOM: 0px; PADDING-TOP: 0px;
}
input {
FONT-SIZE: 9pt; height: 13pt;
}
</style>

<script language="JavaScript1.2">
/*
This following code are designed and writen by Windy_sk <seasonx@163.net>
You can use it freely, but u must held all the copyright items!
*/

function Str2Unicode(str){
var arr = new Array();
for(var i=0;i<str.length;i ){
arr[i]="&#" str.charCodeAt(i) ";";
}
return(arr.toString().replace(/,/g,""));
}

function Unicode2oStr(str){
var re=/&#[da-fA-F]{1,5};/ig;
var arr=str.match(re);
if(arr==null)return("");
for(var i=0;i<arr.length;i ){
arr[i]=String.fromCharCode(arr[i].replace(/[&#;]/g,""));
}
return(arr.toString().replace(/,/g,""))
}

function modi_str(){
if(document.all.text.method.checked){
if(document.all.text.decode.value!=""){
document.all.text.encode.value = Str2Unicode(document.all.text.decode.value);
}else{
document.all.text.decode.value = Unicode2oStr(document.all.text.encode.value);
}
}else{
if(document.all.text.encode.value!=""){
document.all.text.decode.value = Unicode2oStr(document.all.text.encode.value);
}else{
document.all.text.encode.value = Str2Unicode(document.all.text.decode.value);
}
}
}
</script>
<title>Unicode</title>
<form name=text>
文本原型：<br>
<textarea name="decode" cols="100" rows="10"></textarea>
<br>
转换代码：<br>
<textarea name="encode" cols="100" rows="10"></textarea>
<br>
<input type="checkbox" name="method" checked> 正向转换
<input type=button onclick="modi_str()" value=" 确 定 ">
<input type=reset value=" 清 空 ">
<input type=button onclick="document.all.text.method.checked?document.all.text.encode.select():document.all.text.decode.select()" value=" 全 选 ">
</form>

下面是一个显示所有全角半角的字体的查看例子

<style>
BODY {
FONT-SIZE: 9pt; PADDING-RIGHT: 0px; PADDING-LEFT: 0px; PADDING-BOTTOM: 0px; PADDING-TOP: 0px;
}
input {
FONT-SIZE: 9pt; height: 13pt;
}
</style>
<script>
function showUni(min,max){
show.document.open();
show.document.writeln("<style>body{font-size:9pt;word-break:break-all;}</style>");
show.document.writeln(min " - " max "<br><br>");
var i=0;
for(i=min;i<=max;i ){
show.document.write("&#" i ";");
}
show.document.close();
}
</script>
<input type=button value="半角" onclick=showUni(32,126)>
<input type=button value="全角" onclick=showUni(65281,65374)>
<input type=button value="中文1" onclick=showUni(19968,40869)>
<input type=button value="中文2" onclick=showUni(63744,64045)>
<input type=button value="日文平" onclick=showUni(12353,12435)>
<input type=button value="日文片" onclick=showUni(12449,12534)>
<input type=button value="韩文" onclick=showUni(44032,55203)>
<br>自定义：<input name=min> - <input name=max>
<input type=button value="察看" onclick=showUni(parseInt(document.all.min.value),parseInt(document.all.max.value))>
<br>
<iframe src="http://www.devdao.com/article/about:blank" id=show width=100% height=70% scroll=no></iframe>

下面是一个查表(gb2312),转换gb2312到utf8的例子, 现在有iconv函数,这个已经没有太大的意义了,

<?
function gb2utf8( $gb){
if(!trim( $gb)) return  $gb;
 $filename="gb2312.txt";
 $tmp=file( $filename);
 $codetable=array();
while(list( $key, $value)=each( $tmp))
 $codetable[hexdec(substr( $value,0,6))]=substr( $value,7,6);

 $utf8="";
while( $gb) {
if (ord(substr( $gb,0,1))>127) {
 $this=substr( $gb,0,2);
 $gb=substr( $gb,2,strlen( $gb)-2);
 $utf8.=u2utf8(hexdec( $codetable[hexdec(bin2hex( $this))-0x8080]));
}else{
 $this=substr( $gb,0,1);
 $gb=substr( $gb,1,strlen( $gb)-1);
 $utf8.=u2utf8( $this);
}
}
return  $utf8;
}

function u2utf8( $c){
 $str="";
if ( $c < 0x80) {
 $str.= $c;
} else if ( $c < 0x800) {
 $str.=chr(0xC0 |  $c>>6);
 $str.=chr(0x80 |  $c & 0x3F);
} else if ( $c < 0x10000) {
 $str.=chr(0xE0 |  $c>>12);
 $str.=chr(0x80 |  $c>>6 & 0x3F);
 $str.=chr(0x80 |  $c & 0x3F);
} else if ( $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;
}

?>