PHP字符截取函数，兼容各类gbk,utf-8编码

function CsubStrPro($str, $start, $length, $charset = "utf-8", $suffix = false)
{
    if (function_exists ( "mb_substr" ))
        return mb_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 = join ( "", array_slice ( $match [0], $start, $length ) );
    if ($suffix)
        return $slice . "…";

    return $slice;
}

 function subString_UTF8($str, $start, $lenth)
    {
        $len = strlen($str);
        $r = array();
        $n = 0;
        $m = 0;
        for($i = 0; $i < $len; $i++) {
            $x = substr($str, $i, 1);
            $a  = base_convert(ord($x), 10, 2);
            $a = substr('00000000'.$a, -8);
            if ($n < $start){
                if (substr($a, 0, 1) == 0) {
                }elseif (substr($a, 0, 3) == 110) {
                    $i += 1;
                }elseif (substr($a, 0, 4) == 1110) {
                    $i += 2;
                }
                $n++;
            }else{
                if (substr($a, 0, 1) == 0) {
                    $r[ ] = substr($str, $i, 1);
                }elseif (substr($a, 0, 3) == 110) {
                    $r[ ] = substr($str, $i, 2);
                    $i += 1;
                }elseif (substr($a, 0, 4) == 1110) {
                    $r[ ] = substr($str, $i, 3);
                    $i += 2;
                }else{
                    $r[ ] = '';
                }
                if (++$m >= $lenth){
                    break;
                }
            }
        }
        return $r;
    } // End subString_UTF8;
}// End String