How to remove HTML tags in PHP?

<?php
$text = &#39;<p>Test paragraph.</p><!-- Comment --> <a href="#fragment">Other text</a>&#39;;
echo strip_tags($text);
echo "\n";
// 允许 <p> 和 <a>
echo strip_tags($text, &#39;<p><a>&#39;);
?>

Test paragraph. Other text
<p>Test paragraph.</p> <a href="#fragment">Other text</a>

<?php
function strip_tags_content($text, $tags = &#39;&#39;, $invert = FALSE) {

  preg_match_all(&#39;/<(.+?)[\s]*\/?[\s]*>/si&#39;, trim($tags), $tags);
  $tags = array_unique($tags[1]);
   
  if(is_array($tags) AND count($tags) > 0) {
    if($invert == FALSE) {
      return preg_replace(&#39;@<(?!(?:&#39;. implode(&#39;|&#39;, $tags) .&#39;)\b)(\w+)\b.*?>.*?</\1>@si&#39;, &#39;&#39;, $text);
    }
    else {
      return preg_replace(&#39;@<(&#39;. implode(&#39;|&#39;, $tags) .&#39;)\b.*?>.*?</\1>@si&#39;, &#39;&#39;, $text);
    }
  }
  elseif($invert == FALSE) {
    return preg_replace(&#39;@<(\w+)\b.*?>.*?</\1>@si&#39;, &#39;&#39;, $text);
  }
  return $text;
}
?>

<?php
function stripUnwantedTagsAndAttrs($html_str){
  $xml = new DOMDocument();
//Suppress warnings: proper error handling is beyond scope of example
  libxml_use_internal_errors(true);
//List the tags you want to allow here, NOTE you MUST allow html and body otherwise entire string will be cleared
  $allowed_tags = array("html", "body", "b", "br", "em", "hr", "i", "li", "ol", "p", "s", "span", "table", "tr", "td", "u", "ul");
//List the attributes you want to allow here
  $allowed_attrs = array ("class", "id", "style");
  if (!strlen($html_str)){return false;}
  if ($xml->loadHTML($html_str, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD)){
    foreach ($xml->getElementsByTagName("*") as $tag){
      if (!in_array($tag->tagName, $allowed_tags)){
        $tag->parentNode->removeChild($tag);
      }else{
        foreach ($tag->attributes as $attr){
          if (!in_array($attr->nodeName, $allowed_attrs)){
            $tag->removeAttribute($attr->nodeName);
          }
        }
      }
    }
  }
  return $xml->saveHTML();
}