PHP截取含中文的混合字符串长度的函数

/**
* 截取中文混合字符串指定长度
*
* @param string $string
* @param integer $length
* @param string $etc 超过长度时的省略符
* @param string $charset 字符编码 utf-8 或者 gbk
* @return string
*/
public function truncateCn($string, $length = 80, $etc = '...', $charset = 'utf-8')
{
if ($length == 0) {
return '';
}

if (function_exists('mb_substr')) {
$etc = mb_strlen($string, $charset) > $length ? $etc : '';
return mb_substr($string, 0, $length, $charset) . $etc;
}

if ($charset == '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]/";
}
else {
$pa = "/[\x01-\x7f]|[\xa1-\xff][\xa1-\xff]/";
}
preg_match_all($pa, $string, $t_string);
if (count($t_string[0]) > $length) {
return join('', array_slice($t_string[0], 0, $length)) . $etc;
}

return join('', array_slice($t_string[0], 0, $length));
}