直接上代码：
// 先正常截取一遍.
$res = substr($str,$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<a href="https://www.jb51.cc/tag/jiequ/" target="_blank" class="keywords">截取</a>大约6字节
$next_start = <a href="https://www.jb51.cc/tag/start/" target="_blank" class="keywords">$start</a> + $length; // 初始位置
$next_len = $next_start + 6 <= $strlen ? 6 : $strlen - $next_start;
$next_segm = substr($str,$next_start,$next_len);
// 如果第1字节就不是 完整字符的首字节,再往后截取大约6字节
$prev_start = $start - 6 > 0 ? <a href="https://www.jb51.cc/tag/start/" target="_blank" class="keywords">$start</a> - 6 : 0;
$prev_segm = substr($str,$prev_start,<a href="https://www.jb51.cc/tag/start/" target="_blank" class="keywords">$start</a> - $prev_start);
} // start是负数
else {
// 往前再<a href="https://www.jb51.cc/tag/jiequ/" target="_blank" class="keywords">截取</a>大约6字节
$next_start = $strlen + <a href="https://www.jb51.cc/tag/start/" target="_blank" class="keywords">$start</a> + $length; // 初始位置
$next_len = $next_start + 6 <= $strlen ? 6 : $strlen - $next_start;
$next_segm = substr($str,$next_len);
// 如果第1字节就不是 完整字符的首字节,再往后截取大约6字节.
$start = $strlen + $start;
$prev_start = $start - 6 > 0 ? <a href="https://www.jb51.cc/tag/start/" target="_blank" class="keywords">$start</a> - 6 : 0;
$prev_segm = substr($str,<a href="https://www.jb51.cc/tag/start/" target="_blank" class="keywords">$start</a> - $prev_start);
}
// 判断前6字节是否符合utf8规则
if (preg_match('@^([x80-xBF]{0,5})[xC0-xFD]?@',$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 && 191 >= $ord0) {
// 往后<a href="https://www.jb51.cc/tag/jiequ/" target="_blank" class="keywords">截取</a>,并加在res的前面.
if (preg_match('@[xC0-xFD][x80-xBF]{0,5}$@',$prev_segm,$bytes)) {
if (!empty($bytes[0])) {
$bytes = $bytes[0];
$res = $bytes . $res;
}
}
}
if (strlen($res) < $strlen) {
$res = $res . '...';
}
return $res;
}
