mb_substr( $str, $start, $length, $encoding )

$str，需要截断的字符串

$start，截断开始处，起始处为0

$length，要截取的字数

$encoding，网页编码，如utf-8,GB2312,GBK 

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mb_strlen( $str, $encoding )

$str，要计算长度的字符串

$encoding，网页编码，如utf-8,GB2312,GBK