<?php
function fix_html_tags($input, $single_tags = array()) {
	$result = null;
	$stack = array();//标签栈
	$_single_tags = array('br', 'hr', 'img', 'input');//合理的单标签

	if ($single_tags && is_array($single_tags)) {
		$_single_tags = array_merge($_single_tags, $single_tags);
		$_single_tags = array_map('strtolower', $_single_tags);
		$_single_tags = array_unique($_single_tags);
	}
	//返回标签分隔之后的内容，包括标签本身
	$content = preg_split('/(<[^>]+>)/si', $input, null, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE);

	foreach ($content as $val) {
		//匹配未闭合的自闭合标签 如 <br> <hr> 等
		if (preg_match('/<(\w+)[^\/]*>/si', $val, $m) && in_array(strtolower($m[1]), $_single_tags) ) {
			$result .= "\r\n" . $val;
		}
		//匹配标准书写的自闭合标签，直接返回，不入栈
		else if (preg_match('/<(\w+)[^\/]*\/>/si', $val, $m)) {
			$result .= $val;
		}
		//匹配开始标签，并入栈
		else if (preg_match('/<(\w+)[^\/]*>/si', $val, $m)) {
			$result .= "\r\n" . str_repeat("\t", count($stack)) . $val;
			array_push($stack,  $m[1]);
		}
		//匹配闭合标签，匹配前先判断当前闭合标签是栈顶标签的闭合，优先闭合栈顶标签
		else if (preg_match('/<\/(\w+)[^\/]*>/si', $val, $m)) {
			//出栈，多余的闭合标签直接舍弃
			if (strtolower(end($stack)) == strtolower($m[1])) {
				array_pop($stack);
				$result .= $val;
			}
		} else {
			$result .= $val;
		}
	}

	//倒出所有栈内元素
	while ($stack) {
		$result .= "</".array_pop($stack).">";
		$result .= "\r\n";
	}

	return $result;
}


$str="<div>sdfsdf<span>2<span></span>34534<div>121</span></div>";
echo fix_html_tags($str);
?>