参考：
https://www.cnblogs.com/xsphehe/p/5682004.html
示例：
/**
* 参数说明
* $string 欲截取的字符串
* $sublen 截取的长度
* $start 从第几个字节截取，默认为0
* $code 字符编码，默认UTF-8
*/ // $str="哈哈，帅哥在此！";
// echo cut_str($str, 30, 0, 'gb2312'); // gb2312字符编码
// echo cut_str($str,30,0) //utf-8字符编码
// 参考：https://www.cnblogs.com/xsphehe/p/5682004.html
function cut_str($string, $sublen=30, $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;
}
}