php makes the HTML tag automatically complete the closing function code

Briefly explain 1 some code:
1 a ~ ( < [^ > ]+? > )~si
This regular is a match < ...... > The content of. Simply all of them < The label > .
The second ~ < ([a-z0-9]+)[^/ > ]*?/ > ~si
This regular is a match < ... / > The content of. Is a single closed label such as < br / >
Third - < /([a-z0-9]+)[^/ > ]*? > ~si
This regular is a match < /...... > The content of. That's the closing tag < /a >
The fourth ~ < ([a-z0-9]+)[^/ > ]*? > ~si
matching < ...... > The content of. This is not the same as the first one, this one is the real html tag, because the html tag only has Numbers and letters, such as: < h1 > < /h1 >
The fifth ~ < !--.*?-- > ~si
No need to explain, HTMl's comments
The rest can see the source code, php source annex:

 
<?php 
/** 
*  take HTML, And close automatically  
* 
* param $html 
* 
* param $length 
* 
* param $end 
*/ 
function subHtml($html, $length=50) { 
$result = ''; 
$tagStack = array(); 
$len = 0; 
$contents = preg_split("~(<[^>]+?>)~si", $html, -1, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE); 
foreach($contents as $tag) { 
if (trim($tag) == "") continue; 
if (preg_match("~<([a-z0-9]+)[^/>]*?/>~si", $tag)) { 
$result .= $tag; 
} else if (preg_match("~</([a-z0-9]+)[^/>]*?>~si", $tag, $match)) { 
if ($tagStack[count($tagStack)-1] == $match[1]) { 
array_pop($tagStack); 
$result .= $tag; 
} 
} else if (preg_match("~<([a-z0-9]+)[^/>]*?>~si", $tag, $match)) { 
array_push($tagStack, $match[1]); 
$result .= $tag; 
} else if (preg_match("~<!--.*?-->~si", $tag)) { 
$result .= $tag; 
} else { 
if ($len + mstrlen($tag) < $length) { 
$result .= $tag; 
$len += mstrlen($tag); 
} else { 
$str = msubstr($tag, 0, $length - $len + 1); 
$result .= $str; 
break; 
} 
} 
} while (!empty($tagStack)) { 
$result .= '</' . array_pop($tagStack) . '>'; 
} 
return $result; 
} 
/** 
*  Fetch Chinese string  
* 
* param $string  string  
* 
* param $start  Start bit  
* 
* param $length  The length of the  
* 
* param $charset  coding  
* 
* param $dot  Additional string  
*/ 
function msubstr($string, $start, $length, $dot = '', $charset = 'UTF-8') { 
$string = str_replace(array('&amp;', '&quot;', '&lt;', '&gt;', '&nbsp;'), array('&', '"', '<', '>', ' '), $string); 
if (strlen($string) <= $length) { 
return $string; 
} 
if (strtolower($charset) == 'utf-8') { 
$n = $tn = $noc = 0; 
while ($n < strlen($string)) { 
$t = ord($string[$n]); 
if ($t == 9 || $t == 10 || (32 <= $t && $t <= 126)) { 
$tn = 1; 
$n++; 
} elseif (194 <= $t && $t <= 223) { 
$tn = 2; 
$n += 2; 
} elseif (224 <= $t && $t <= 239) { 
$tn = 3; 
$n += 3; 
} elseif (240 <= $t && $t <= 247) { 
$tn = 4; 
$n += 4; 
} elseif (248 <= $t && $t <= 251) { 
$tn = 5; 
$n += 5; 
} elseif ($t == 252 || $t == 253) { 
$tn = 6; 
$n += 6; 
} else { 
$n++; 
} 
$noc++; 
if ($noc >= $length) { 
break; 
} 
} 
if ($noc > $length) { 
$n -= $tn; 
} 
$strcut = substr($string, 0, $n); 
} else { 
for($i = 0; $i < $length; $i++) { 
$strcut .= ord($string[$i]) > 127 ? $string[$i] . $string[++$i] : $string[$i]; 
} 
} 
return $strcut . $dot; 
} 
/** 
*  Get the length of the string, both in Chinese and English.  
*/ 
function mstrlen($str, $charset = 'UTF-8') { 
if (function_exists('mb_substr')) { 
$length = mb_strlen($str, $charset); 
} elseif (function_exists('iconv_substr')) { 
$length = iconv_strlen($str, $charset); 
} else { 
preg_match_all("/[\x01-\x7f]|[\xc2-\xdf][\x80-\xbf]|\xe0[\xa0-\xbf][\x80-\xbf]|[\xe1-\xef][\x80-\xbf][\x80-\xbf]|\xf0[\x90-\xbf][\x80-f][\x80-\xbf]|[\xf1-\xf7][\x80-\xbf][\x80-\xbf][\x80-\xbf]/", $text, $ar); 
$length = count($ar[0]); 
} 
return $length; 
} 
$str = "<div><table>x<tr>1s<td> test <td>124"; 
echo subHtml($str); 
?>