php截取字符串

一个很好的截取字符串的方法

/**
+----------------------------------------------------------
* 字符串截取，支持中文和其他编码
+----------------------------------------------------------
+----------------------------------------------------------
* @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 $suffix 截断显示字符
+----------------------------------------------------------
* @return string
+----------------------------------------------------------
*/
public function msubstr($str, $start=0, $length, $charset="utf-8", $suffix=true) {
    if(function_exists("mb_substr"))
        $slice = mb_substr($str, $start, $length, $charset);
    elseif(function_exists('iconv_substr')) {
        $slice = iconv_substr($str,$start,$length,$charset);
    }else{
        $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, $length));
    }
    if($suffix && $str != $slice) return $slice."...";
    return $slice;
}