字符串截取函数

<?php

/**
+----------------------------------------------------------
* 字符串截取，支持中文和其他编码
+----------------------------------------------------------
* @static
* @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", $suffix=true)
{
    // 加载php_mbstring扩展时有效
    if (function_exists("mb_substr"))
        return mb_substr($str, $start, $length, $charset);
    // PHP 5+版本有效
    if (function_exists("iconv_substr"))
        return iconv_substr($str, $start, $length, $charset);

    $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);
    $slice = implode("", array_slice($match[0], $start, $length));
    if ($suffix)
	    return $slice."...";
    else
	    return $slice;
}

// for test

$mouse = '我是中国人！';
echo msubstr($mouse, 0, 5);	// print '我是中国人...'