php的mbstring扩展模块提供了多字节字符的处理能力，平常最常用的就是用mbstring来切分多字节的中文字符，这样可以避免出现半个字符的情况，由于是php的扩展，它的性能也要比一些自定义的多字节切分函数要好上一些。

mbstring extension提供了几个功能类似的函数，mb_substr和mb_strcut，看看手册上对它们的解释。

mb_substr
mb_substr() returns the portion of str specified by the start and length parameters.

mb_substr() performs multi-byte safe substr() operation based on number of characters. Position is counted from the beginning of str. First character's position is 0. Second character position is 1, and so on.

mb_strcut
mb_strcut() returns the portion of str specified by the start and length parameters.

mb_strcut() performs equivalent operation as mb_substr() with different method. If start position is multi-byte character's second byte or larger, it starts from first byte of multi-byte character.

It subtracts string from str that is shorter than length AND character that is not part of multi-byte string or not being middle of shift sequence.

举个例子来说,有一段文字, 分别用mb_substr和mb_strcut来做切分:

PLAIN TEXT
CODE:
str = '我是一串比较长的中文-www.dwww.cn';

echo "mb_substr:" . mb_substr(str, 0, 6, 'utf-8');

echo "
";

echo "mb_strcut:" . mb_strcut(str, 0, 6, 'utf-8');
?>
输出结果如下：

mb_substr:我是一串比较
mb_strcut:我是