直接上代码：
/*** 完整词的截取** @param $str* @param $start* @param $length** @return string*/ public static function usubstr($str, $start, $length = null) {// 先正常截取一遍.$res = substr($str, $start, $length);$strlen = strlen($str);/* 接着判断头尾各6字节是否完整(不残缺) */// 如果参数start是正数if ($start >= 0) {// 往前再截取大约6字节$next_start = $start + $length; // 初始位置$next_len = $next_start + 6 0 ? $start - 6 : 0;$prev_segm = substr($str, $prev_start, $start - $prev_start);} // start是负数else {// 往前再截取大约6字节$next_start = $strlen + $start + $length; // 初始位置$next_len = $next_start + 6 0 ? $start - 6 : 0;$prev_segm = substr($str, $prev_start, $start - $prev_start);}// 判断前6字节是否符合utf8规则if (preg_match('@^([x80-xbf]{0,5})[xc0-xfd]?@', $next_segm, $bytes)) {if (!empty($bytes[1])) {$bytes = $bytes[1];$res .= $bytes;}}// 判断后6字节是否符合utf8规则$ord0 = ord($res[0]);if (128 = $ord0) {// 往后截取 , 并加在res的前面.if (preg_match('@[xc0-xfd][x80-xbf]{0,5}$@', $prev_segm, $bytes)) {if (!empty($bytes[0])) {$bytes = $bytes[0];$res = $bytes . $res;}}}if (strlen($res) ，