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来自PHPCMS

/**
 * 字符截取 支持UTF8/GBK
 * @param $string
 * @param $length
 * @param $dot
 */
function str_cut($string, $length, $dot = '...') {
	$strlen = strlen($string);
	if($strlen <= $length) return $string;
	$string = str_replace(array(' ',' ', '&', '"', ''', '“', '”', '—', '<', '>', '·', '…'), array('∵',' ', '&', '"', "'", '“', '”', '—', '<', '>', '·', '…'), $string);
	$strcut = '';
	if(strtolower(CHARSET) == 'utf-8') {
		$length = intval($length-strlen($dot)-$length/3);
		$n = $tn = $noc = 0;
		while($n < strlen($string)) {
			$t = ord($string[$n]);
			if($t == 9 || $t == 10 || (32 <= $t && $t <= 126)) {
				$tn = 1; $n++; $noc++;
			} elseif(194 <= $t && $t <= 223) {
				$tn = 2; $n += 2; $noc += 2;
			} elseif(224 <= $t && $t <= 239) {
				$tn = 3; $n += 3; $noc += 2;
			} elseif(240 <= $t && $t <= 247) {
				$tn = 4; $n += 4; $noc += 2;
			} elseif(248 <= $t && $t <= 251) {
				$tn = 5; $n += 5; $noc += 2;
			} elseif($t == 252 || $t == 253) {
				$tn = 6; $n += 6; $noc += 2;
			} else {
				$n++;
			}
			if($noc >= $length) {
				break;
			}
		}
		if($noc > $length) {
			$n -= $tn;
		}
		$strcut = substr($string, 0, $n);
		$strcut = str_replace(array('∵', '&', '"', "'", '“', '”', '—', '<', '>', '·', '…'), array(' ', '&', '"', ''', '“', '”', '—', '<', '>', '·', '…'), $strcut);
	} else {
		$dotlen = strlen($dot);
		$maxi = $length - $dotlen - 1;
		$current_str = '';
		$search_arr = array('&',' ', '"', "'", '“', '”', '—', '<', '>', '·', '…','∵');
		$replace_arr = array('&',' ', '"', ''', '“', '”', '—', '<', '>', '·', '…',' ');
		$search_flip = array_flip($search_arr);
		for ($i = 0; $i < $maxi; $i++) {
			$current_str = ord($string[$i]) > 127 ? $string[$i].$string[++$i] : $string[$i];
			if (in_array($current_str, $search_arr)) {
				$key = $search_flip[$current_str];
				$current_str = str_replace($search_arr[$key], $replace_arr[$key], $current_str);
			}
			$strcut .= $current_str;
		}
	}
	return $strcut.$dot;
}