现在很多情况下，需要用到截取字符串功能，不够由于gb2312与utf-8的编码问题，需要区别对待，下面是具体的实现方法并从一些比较成熟的系统中扒下的代码。

1、UTF-8、GB2312都支持的汉字截取函数

 
<?php 
/* 
Utf-8、gb2312都支持的汉字截取函数 
cut_str(字符串, 截取长度, 开始长度, 编码); 
编码默认为 utf-8 
开始长度默认为 0 
*/ 
function cut_str($string, $sublen, $start = 0, $code = 'UTF-8') 
{ 
if($code == 'UTF-8') 
{ 
$pa = "/[\x01-\x7f]|[\xc2-\xdf][\x80-\xbf]|\xe0[\xa0-\xbf][\x80-\xbf]|[\xe1-\xef][\x80-\xbf][\x80-\xbf]|\xf0[\x90-\xbf][\x80-\xbf][\x80-\xbf]|[\xf1-\xf7][\x80-\xbf][\x80-\xbf][\x80-\xbf]/"; 
preg_match_all($pa, $string, $t_string); 
if(count($t_string[0]) - $start > $sublen) return join('', array_slice($t_string[0], $start, $sublen))."…"; 
return join('', array_slice($t_string[0], $start, $sublen)); 
} 
else 
{ 
$start = $start*2; 
$sublen = $sublen*2; 
$strlen = strlen($string); 
$tmpstr = ''; 
for($i=0; $i< $strlen; $i++) 
{ 
if($i>=$start && $i< ($start+$sublen)) 
{ 
if(ord(substr($string, $i, 1))>129) 
{ 
$tmpstr.= substr($string, $i, 2); 
} 
else 
{ 
$tmpstr.= substr($string, $i, 1); 
} 
} 
if(ord(substr($string, $i, 1))>129) $i++; 
} 
if(strlen($tmpstr)< $strlen ) $tmpstr.= "…"; 
return $tmpstr; 
} 
} 
$str = "abcd需要截取的字符串"; 
echo cut_str($str, 8, 0, 'gb2312'); 
?> 

2、截取utf8编码的多字节字符串

 
<?php 
//截取utf8字符串 
function utf8Substr($str, $from, $len) 
{ 
return preg_replace('#^(?:[\x00-\x7F]|[\xC0-\xFF][\x80-\xBF]+){0,'.$from.'}'. 
'((?:[\x00-\x7F]|[\xC0-\xFF][\x80-\xBF]+){0,'.$len.'}).*#s', 
'$1',$str); 
} 
?> 

3、截取GB2312中文字符串

 
<?php 
//截取中文字符串 
function mysubstr($str, $start, $len) { 
$tmpstr = ""; 
$strlen = $start + $len; 
for($i = 0; $i < $strlen; $i++) { 
if(ord(substr($str, $i, 1)) > 0xa0) { 
$tmpstr .= substr($str, $i, 2); 
$i++; 
} else 
$tmpstr .= substr($str, $i, 1); 
} 
return $tmpstr; 
} 
?> 

4、BugFree 的字符截取函数

 
<?php 
/** 
* @package BugFree 
* @version $Id: FunctionsMain.inc.php,v 1.32 2005/09/24 11:38:37 wwccss Exp $ 
* 
* 
* Return part of a string(Enhance the function substr()) 
* 
* @param string $String the string to cut. 
* @param int $Length the length of returned string. 
* @param booble $Append whether append "…": false|true 
* @return string the cutted string. 
*/ 
function sysSubStr($String,$Length,$Append = false) 
{ 
if (strlen($String) < = $Length ) 
{ 
return $String; 
} 
else 
{ 
$I = 0; 
while ($I < $Length) 
{ 
$StringTMP = substr($String,$I,1); 
if ( ord($StringTMP) >=224 ) 
{ 
$StringTMP = substr($String,$I,3); 
$I = $I + 3; 
} 
elseif( ord($StringTMP) >=192 ) 
{ 
$StringTMP = substr($String,$I,2); 
$I = $I + 2; 
} 
else 
{ 
$I = $I + 1; 
} 
$StringLast[] = $StringTMP; 
} 
$StringLast = implode("",$StringLast); 
if($Append) 
{ 
$StringLast .= "…"; 
} 
return $StringLast; 
} 
} 
$String = "http://www.jb51.net ― 简单、精彩、通用"; 
$Length = "18"; 
$Append = false; 
echo sysSubStr($String,$Length,$Append); 
?> 

dedecms中的截取代码
这是从dedecms直接拿的代码，大家可以稍作修改即可。

 
//中文截取2，单字节截取模式 
//如果是request的内容，必须使用这个函数 
function cn_substrR($str,$slen,$startdd=0) 
{ 
$str = cn_substr(stripslashes($str),$slen,$startdd); 
return addslashes($str); 
} 
//中文截取2，单字节截取模式 
function cn_substr($str,$slen,$startdd=0) 
{ 
global $cfg_soft_lang; 
if($cfg_soft_lang=='utf-8') 
{ 
return cn_substr_utf8($str,$slen,$startdd); 
} 
$restr = ''; 
$c = ''; 
$str_len = strlen($str); 
if($str_len < $startdd+1) 
{ 
return ''; 
} 
if($str_len < $startdd + $slen || $slen==0) 
{ 
$slen = $str_len - $startdd; 
} 
$enddd = $startdd + $slen - 1; 
for($i=0;$i<$str_len;$i++) 
{ 
if($startdd==0) 
{ 
$restr .= $c; 
} 
else if($i > $startdd) 
{ 
$restr .= $c; 
} 
if(ord($str[$i])>0x80) 
{ 
if($str_len>$i+1) 
{ 
$c = $str[$i].$str[$i+1]; 
} 
$i++; 
} 
else 
{ 
$c = $str[$i]; 
} 
if($i >= $enddd) 
{ 
if(strlen($restr)+strlen($c)>$slen) 
{ 
break; 
} 
else 
{ 
$restr .= $c; 
break; 
} 
} 
} 
return $restr; 
} 
//utf-8中文截取，单字节截取模式 
function cn_substr_utf8($str, $length, $start=0) 
{ 
if(strlen($str) < $start+1) 
{ 
return ''; 
} 
preg_match_all("/./su", $str, $ar); 
$str = ''; 
$tstr = ''; 
//为了兼容mysql4.1以下版本,与数据库varchar一致,这里使用按字节截取 
for($i=0; isset($ar[0][$i]); $i++) 
{ 
if(strlen($tstr) < $start) 
{ 
$tstr .= $ar[0][$i]; 
} 
else 
{ 
if(strlen($str) < $length + strlen($ar[0][$i]) ) 
{ 
$str .= $ar[0][$i]; 
} 
else 
{ 
break; 
} 
} 
} 
return $str; 
} 

phpcms中的字符串截取代码:

 
function str_cut($string, $length, $dot = '...') 
{ 
$strlen = strlen($string); 
if($strlen <= $length) return $string; 
$string = str_replace(array(' ', '&', '"', ''', '“', '”', '—', '<', '>', '·', '…'), array(' ', '&', '"', "'", '“', '”', '―', '<', '>', '・', '…'), $string); 
$strcut = ''; 
if(strtolower(CHARSET) == 'utf-8') 
{ 
$n = $tn = $noc = 0; 
while($n < $strlen) 
{ 
$t = ord($string[$n]); 
if($t == 9 || $t == 10 || (32 <= $t && $t <= 126)) { 
$tn = 1; $n++; $noc++; 
} elseif(194 <= $t && $t <= 223) { 
$tn = 2; $n += 2; $noc += 2; 
} elseif(224 <= $t && $t < 239) { 
$tn = 3; $n += 3; $noc += 2; 
} elseif(240 <= $t && $t <= 247) { 
$tn = 4; $n += 4; $noc += 2; 
} elseif(248 <= $t && $t <= 251) { 
$tn = 5; $n += 5; $noc += 2; 
} elseif($t == 252 || $t == 253) { 
$tn = 6; $n += 6; $noc += 2; 
} else { 
$n++; 
} 
if($noc >= $length) break; 
} 
if($noc > $length) $n -= $tn; 
$strcut = substr($string, 0, $n); 
} 
else 
{ 
$dotlen = strlen($dot); 
$maxi = $length - $dotlen - 1; 
for($i = 0; $i < $maxi; $i++) 
{ 
$strcut .= ord($string[$i]) > 127 ? $string[$i].$string[++$i] : $string[$i]; 
} 
} 
$strcut = str_replace(array('&', '"', "'", '<', '>'), array('&', '"', ''', '<', '>'), $strcut); 
return $strcut.$dot; 
}