La clase DOMDocument

Showing a quick example of how to use this class, just so that new users can get a quick start without having to figure it all out by themself. ( At the day of posting, this documentation just got added and is lacking examples. )
<?php
// Set the content type to be XML, so that the browser will recognise it as XML.
header( "content-type: application/xml; charset=ISO-8859-15" );
// "Create" the document.
$xml = new DOMDocument( "1.0", "ISO-8859-15" );
// Create some elements.
$xml_album = $xml->createElement( "Album" );
$xml_track = $xml->createElement( "Track", "The ninth symphony" );
// Set the attributes.
$xml_track->setAttribute( "length", "0:01:15" );
$xml_track->setAttribute( "bitrate", "64kb/s" );
$xml_track->setAttribute( "channels", "2" );
// Create another element, just to show you can add any (realistic to computer) number of sublevels.
$xml_note = $xml->createElement( "Note", "The last symphony composed by Ludwig van Beethoven." );
// Append the whole bunch.
$xml_track->appendChild( $xml_note );
$xml_album->appendChild( $xml_track );
// Repeat the above with some different values..
$xml_track = $xml->createElement( "Track", "Highway Blues" );
$xml_track->setAttribute( "length", "0:01:33" );
$xml_track->setAttribute( "bitrate", "64kb/s" );
$xml_track->setAttribute( "channels", "2" );
$xml_album->appendChild( $xml_track );
$xml->appendChild( $xml_album );
// Parse the XML.
print $xml->saveXML();
?>

Output:
<Album>
 <Track length="0:01:15" bitrate="64kb/s" channels="2">
 The ninth symphony
 <Note>
 The last symphony composed by Ludwig van Beethoven.
 </Note>
 </Track>
 <Track length="0:01:33" bitrate="64kb/s" channels="2">Highway Blues</Track>
</Album>
If you want your PHP->DOM code to run under the .xml extension, you should set your webserver up to run the .xml extension with PHP ( Refer to the installation/configuration configuration for PHP on how to do this ).
Note that this:
<?php
$xml = new DOMDocument( "1.0", "ISO-8859-15" );
$xml_album = $xml->createElement( "Album" );
$xml_track = $xml->createElement( "Track" );
$xml_album->appendChild( $xml_track );
$xml->appendChild( $xml_album );
?>

is NOT the same as this:
<?php
// Will NOT work.
$xml = new DOMDocument( "1.0", "ISO-8859-15" );
$xml_album = new DOMElement( "Album" );
$xml_track = new DOMElement( "Track" );
$xml_album->appendChild( $xml_track );
$xml->appendChild( $xml_album );
?>

although this will work:
<?php
$xml = new DOMDocument( "1.0", "ISO-8859-15" );
$xml_album = new DOMElement( "Album" );
$xml->appendChild( $xml_album );
?>

For those landing here and checking for encoding issue with utf-8 characteres, it's pretty easy to correct it, without adding any additional output tag to your html.
We'll be utilizing: mb_convert_encoding
Thanks to the user who shared: SmartDOMDocument in previous comments, I got the idea of solving it. However I truly wish that he shared the method instead of giving a link.
Anyway coming back to the solution, you can simply use:
<?php
 // checks if the content we're receiving isn't empty, to avoid the warning
 if ( empty( $content ) ) {
 return false;
 }
 // converts all special characters to utf-8
 $content = mb_convert_encoding($content, 'HTML-ENTITIES', 'UTF-8');
 // creating new document
 $doc = new DOMDocument('1.0', 'utf-8');
 //turning off some errors
 libxml_use_internal_errors(true);
 // it loads the content without adding enclosing html/body tags and also the doctype declaration
 $doc->LoadHTML($content, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
 // do whatever you want to do with this code now
?>

I hope it solves the issue for someone! If you need my help or service to fix your code, you can reach me on nabtron.com or contact me at the email mentioned with this comment.

After struggling with parsing and modifying partial HTML content for several hours, I came to this solution which does work for me and is relatively simple compared to what else I found online.
This solution fixes unwanted DOCTYPE and html, body tags as well as encoding issues.
<?php
// Assumption: content is utf-8 encoded
$content = "<h1>This is a heading</h1><p>This is a paragraph</p>";
// Load content to a div and specify encoding with a meta tag
$temp_dom = new DOMDocument();
$temp_dom->loadHTML("<meta http-equiv='Content-Type' content='charset=utf-8' /><div>$content</div>");
// As loadHTML() adds a DOCTYPE as well as <html> and <body> tag, let’s create another DOMDocument and import just the nodes we want
$dom = new DOMDocument();
$first_div = $temp_dom->getElementsByTagName('div')[0];
$first_div_node = $dom->importNode($first_div, true);
$dom->appendChild($first_div_node);
// Do whatever you want to do
$dom->getElementsByTagName('h1')[0]->setAttribute('class', 'happy');
// You could also just echo $dom->saveHtml() if you don’t mind the div and whitespace 
echo substr(trim($dom->saveHtml()), 5, -6);
// Outputs: <h1 class="happy">This is a heading</h1><p>This is a paragraph</p>
?>

Here's a small function I wrote to get all page links using the DOMDocument which will hopefully be of use to others
<?php
/**
 * @author Jay Gilford
 */
 
/**
 * get_links()
 * 
 * @param string $url
 * @return array
 */
function get_links($url) {
 
 // Create a new DOM Document to hold our webpage structure
 $xml = new DOMDocument();
 
 // Load the url's contents into the DOM
 $xml->loadHTMLFile($url);
 
 // Empty array to hold all links to return
 $links = array();
 
 //Loop through each <a> tag in the dom and add it to the link array
 foreach($xml->getElementsByTagName('a') as $link) {
 $links[] = array('url' => $link->getAttribute('href'), 'text' => $link->nodeValue);
 }
 
 //Return the links
 return $links;
}
?>

For anyone else who has been having issues with formatOuput not working, here is a work-around:
rather than just doing something like:
<?php
$outXML = $xml->saveXML();
?>

force it to reload the XML from scratch, then it will format correctly:
<?php
$outXML = $xml->saveXML();
$xml = new DOMDocument();
$xml->preserveWhiteSpace = false;
$xml->formatOutput = true;
$xml->loadXML($outXML);
$outXML = $xml->saveXML();
?>

Look out for the following gotcha when loading XML from a string:
<?php
$doc = new \DOMDocument;
$doc->documentURI = $myXmlFilename;
$doc->loadXML($myXmlString);
?>

documentURI is now set to the value of $myXmlFilename, right?
Wrong!
It's set to the current working directory. If you want to manually set documentURI to something other than the CWD, do so AFTER the call to loadXML().
E.g.:
<?php
$doc = new \DOMDocument;
$doc->loadXML($myXmlString);
$doc->documentURI = $myXmlFilename;
?>

documentURI really is now set to the value of $myXmlFilename.

You may need to save all or part of a DOMDocument as an XHTML-friendly string, something compliant with both XML and HTML 4. Here's the DOMDocument class extended with a saveXHTML method:
<?php
/**
 * XHTML Document
 *
 * Represents an entire XHTML DOM document; serves as the root of the document tree.
 */
class XHTMLDocument extends DOMDocument {
 /**
 * These tags must always self-terminate. Anything else must never self-terminate.
 * 
 * @var array
 */
 public $selfTerminate = array(
 'area','base','basefont','br','col','frame','hr','img','input','link','meta','param'
 );
 
 /**
 * saveXHTML
 *
 * Dumps the internal XML tree back into an XHTML-friendly string.
 *
 * @param DOMNode $node
 * Use this parameter to output only a specific node rather than the entire document.
 */
 public function saveXHTML(DOMNode $node=null) {
 
 if (!$node) $node = $this->firstChild;
 
 $doc = new DOMDocument('1.0');
 $clone = $doc->importNode($node->cloneNode(false), true);
 $term = in_array(strtolower($clone->nodeName), $this->selfTerminate);
 $inner='';
 
 if (!$term) {
 $clone->appendChild(new DOMText(''));
 if ($node->childNodes) foreach ($node->childNodes as $child) {
 $inner .= $this->saveXHTML($child);
 }
 }
 
 $doc->appendChild($clone);
 $out = $doc->saveXML($clone);
 
 return $term ? substr($out, 0, -2) . ' />' : str_replace('><', ">$inner<", $out);
 }
}
?>

This hasn't been benchmarked, but is probably significantly slower than saveXML or saveHTML and should be used sparingly.

For using safely with script nodes when parsing, best option is extending DOMDocument, keeping script tags while DOMDocument process and rearrange them just after saveHTML function is called. Here is my custom class.
<?php 
class SafeDOMDocument extends \DOMDocument
{
 const REGEX_JS = '#(\s*<!--(\[if[^\n]*>)?\s*(<script.*</script>)+\s*(<!\[endif\])?-->)|(\s*<script.*</script>)#isU';
 const SUBSTITUTION_FORMAT = '<!--<script class="script_%s"></script>-->';
 private $matchedScripts = [];
 public function loadHTML($source, $options = 0)
 {
 $this->formatOutput = false;
 $this->preserveWhiteSpace = true;
 $this->validateOnParse = false;
 $this->strictErrorChecking = false;
 $this->recover = false;
 $this->resolveExternals = false;
 $this->substituteEntities = false;
 $matches = [];
 $success = preg_match_all(self::REGEX_JS, $source, $matches);
 if ($success && !empty($matches)) {
 foreach ($matches[0] as $match) {
 $storedScript = rtrim(ltrim($match, "\n\r\t "), "\n\r\t ");
 $scriptId = md5($storedScript);
 $key = sprintf(self::SUBSTITUTION_FORMAT, $scriptId);
 $source = str_replace($match, $key, $source);
 $this->matchedScripts[$key] = $storedScript;
 }
 }
 return parent::loadHTML($source, $options);
 }
 public function saveHTML(DOMNode $node = null)
 {
 $output = parent::saveHTML($node);
 if (count($this->matchedScripts)) {
 foreach ($this->matchedScripts as $substitution => $originalSnippet) {
 $output = str_replace($substitution, $originalSnippet, $output);
 }
 }
 return $output;
 }
}
?>

Be careful with formatOutput().
Creating an empty node like this:
createElement('foo','')
instead of
createElement('foo')
will break formatOutput.

A nice and simple node 2 array I wrote, worth a try ;) 
<?php
function getArray($node)
{
 $array = false;
 if ($node->hasAttributes())
 {
 foreach ($node->attributes as $attr)
 {
 $array[$attr->nodeName] = $attr->nodeValue;
 }
 }
 if ($node->hasChildNodes())
 {
 if ($node->childNodes->length == 1)
 {
 $array[$node->firstChild->nodeName] = $node->firstChild->nodeValue;
 }
 else
 {
 foreach ($node->childNodes as $childNode)
 {
 if ($childNode->nodeType != XML_TEXT_NODE)
 {
 $array[$childNode->nodeName][] = $this->getArray($childNode);
 }
 }
 }
 }
 return $array;
}
?>

While DOMDocument can technically be used to parse HTML, it is not ideal for HTML documents and is better suited for processing well-formed XML. One of the primary issues with using DOMDocument for HTML is its strict handling of special characters, such as the ampersand (&).
DOMDocument requires that ampersands be escaped as &, which is in line with XML standards but can be counterintuitive for handling real-world HTML, where raw & characters are commonly found, especially in URLs and text. This behavior stems from the underlying XML-based parser (libxml), which treats HTML with the same strictness as XML.
This problem has been reported as far back as 2001, yet the same parsing errors continue to occur when using DOMDocument on HTML documents today.
A common workaround developers use is to suppress the error reporting from DOMDocument, particularly when parsing errors like unescaped ampersands occur. However, suppressing these errors is not recommended, especially in production environments, as it can hide important issues and pose potential security risks. Ignoring or suppressing errors can leave warnings unnoticed, which may result in vulnerabilities if not properly addressed.
For these reasons, it's advisable to use DOMDocument primarily for XML documents, or to consider more appropriate libraries when working with HTML to avoid these issues.
theCoder / MV

This function may help to debug current dom element:
<?php
function dom_dump($obj) {
 if ($classname = get_class($obj)) {
 $retval = "Instance of $classname, node list: \n";
 switch (true) {
 case ($obj instanceof DOMDocument):
 $retval .= "XPath: {$obj->getNodePath()}\n".$obj->saveXML($obj);
 break;
 case ($obj instanceof DOMElement):
 $retval .= "XPath: {$obj->getNodePath()}\n".$obj->ownerDocument->saveXML($obj);
 break;
 case ($obj instanceof DOMAttr):
 $retval .= "XPath: {$obj->getNodePath()}\n".$obj->ownerDocument->saveXML($obj);
 //$retval .= $obj->ownerDocument->saveXML($obj);
 break;
 case ($obj instanceof DOMNodeList):
 for ($i = 0; $i < $obj->length; $i++) {
 $retval .= "Item #$i, XPath: {$obj->item($i)->getNodePath()}\n".
"{$obj->item($i)->ownerDocument->saveXML($obj->item($i))}\n";
 }
 break;
 default:
 return "Instance of unknown class";
 }
 } else {
 return 'no elements...';
 }
 return htmlspecialchars($retval);
}
?>

Example usage:
<?php
$dom = new DomDocument();
$dom->load('test.xml');
$body = $dom->documentElement->getElementsByTagName('book');
echo '<pre>'.dom_dump($body).'<pre>';
?>

Output:
Instance of DOMNodeList, node list: 
Item #0, XPath: /library/book[1]
<book isbn="0345342968">
<title>Fahrenheit 451</title>
<author>R. Bradbury</author>
<publisher>Del Rey</publisher>
</book>
Item #1, XPath: /library/book[2]
<book isbn="0048231398">
<title>The Silmarillion</title>
<author>J.R.R. Tolkien</author>
<publisher>G. Allen &amp; Unwin</publisher>
</book>
Item #2, XPath: /library/book[3]
<book isbn="0451524934">
<title>1984</title>
<author>G. Orwell</author>
<publisher>Signet</publisher>
</book>
Item #3, XPath: /library/book[4]
<book isbn="031219126X">
<title>Frankenstein</title>
<author>M. Shelley</author>
<publisher>Bedford</publisher>
</book>
Item #4, XPath: /library/book[5]
<book isbn="0312863551">
<title>The Moon Is a Harsh Mistress</title>
<author>R. A. Heinlein</author>
<publisher>Orb</publisher>
</book>

How to objetify a DomDocument with hierarchy like:
<root>
 <item>
 <prop1>info1</prop1>
 <prop2>info2</prop2>
 <prop3>info3</prop3>
 </item>
 <item>
 <prop1>info1</prop1>
 <prop2>info2</prop2>
 <prop3>info3</prop3>
 </item>
</root>
It's possible to use in object style to retrieve information, as:
<?php
 $theNodeValue = $aitem->prop1;
?>

Here is the code: one Class and 2 functions.
<?php
 class ArrayNode{
 public $nodeName, $nodeValue;
 }
 function getChildNodeElements( $domNode ){
 $nodes = array();
 for( $i=0; $i < $domNode->childNodes->length; $i++){
 $cn = $domNode->childNodes->item($i);
 if( $cn->nodeType == 1){
 $nodes[] = $cn;
 }
 }
 return $nodes;
 }
 function getArrayNodes( $domDoc ){
 $res = array();
 for( $i=0; $i < $domDoc->childNodes->length; $i++){
 $cn = $domDoc->childNodes->item($i);
 # The first is the root tag...
 if( $cn->nodeType == 1){
 # But we want it's childNodes.
 $sub_cn = getChildNodeElements( $cn);
 # Found the tagName:
 $baseItemTagName = $sub_cn[0]->nodeName;
 break;
 }
 }
 $dnl = $domDoc->getElementsByTagName( $baseItemTagName);
 for( $i=0; $i< $dnl->length; $i++){
 $arrayNode = new ArrayNode();
 # Summary
 $arrayNode->nodeName = $dnl->item($i)->nodeName;
 $arrayNode->nodeValue = $dnl->item($i)->nodeValue;
 # Child Nodes
 $cn = $dnl->item($i)->childNodes;
 for( $k=0; $k<$cn->length; $k++){
 if( $cn->item($k)->nodeName == "#text" && trim($cn->item($k)->nodeValue) == "") continue;
 $arrayNode->{$cn->item($k)->nodeName} = $cn->item($k)->nodeValue;
 }
 # Attributes
 $attr = $dnl->item($i)->attributes;
 for( $k=0; $k < $attr->length; $k++){
 if(! is_null($attr)){
 if( $attr->item($k)->nodeName == "#text" && trim($attr->item($k)->nodeValue) == "") continue;
 $arrayNode->{$attr->item($k)->nodeName} = $attr->item($k)->nodeValue;
 }
 }
 $res[] = $arrayNode;
 }
 return $res;
 }
?>

To use it:
<?php
 # First you load a XML in a DomDocument variable.
 $url = "/path/to/yourxmlfile.xml";
 $domSrc = file_get_contents($url);
 $dom = new DomDocument();
 $dom->loadXML( $domSrc );
 # Then, you get the ArrayNodes from the DomDocument.
 $ans = getArrayNodes( $dom );
 
 for( $i=0; $i < count( $ans ) ; $i++){
 $cn = $ans[ $i];
 $info1 = $cn->prop1;
 $info2 = $cn->prop2;
 $info3 = $cn->prop3;
 
 // ...
 
 }
?>

when you add the new element to formatted XML data through appendChild() method, you would the new element you add is not be formatted(that is not indexed, not line break). here is my solution (in short load the xml without preserve white space, ), example show as below:
<?php
$doc = new \DOMDocument();
$doc->formatOutput = true;
$doc->preserveWhiteSpace = false;//that is key, default value is true. 
$doc->loadXML($xmlStr);
$doc->appendChild($doc->createElement('php', '666'))
$formattedXMLStr = $doc->saveXML();//DOMDocument wold format the xml str for you
echo $formattedXMlStr;
?>
it take me some time to try it out. hope it save your time.

/* Function evolved from jay at jaygilford dot com post
 * This function will return an array of the values of the specified
 * attribute ($attr) for all the Dom Document object's elements 
 */
<?php
function getAttrData(string $attr, DomDocument $dom) { 
 // Empty array to hold all classes to return 
 $attrData = array(); 
 //Loop through each tag in the dom and add it's attribute data to the array 
 foreach($dom->getElementsByTagName('*') as $tag) {
 if(empty($tag->getAttribute($attr)) === false) {
 array_push($attrData, $tag->getAttribute($attr));
 }
 } 
 //Return the array of attribute data
 return array_unique($attrData); 
}
$html = '
<!DOCTYPE html>
<html>
<head>
<title>Page Title</title>
</head>
<body>
<a href="#someLink" id="someLink" class="link-class">Some Link</a>
<a href="#someOtherLink" id="someOtherLink" class="link-class">Some Other Link</a>
<h1 id="header1" class="header-class">My First Heading</h1>
<p id="para1" class="para-class">My first paragraph.</p>
</body>
</html>';
$dom = new DOMDocument();
$dom->loadHtml($html);
$dom->saveHTML();
var_dump(getAttrData('class', $dom));

Easy function for basic output of XML file via DOM parsing
<?php
$dom = new DomDocument();
$dom->load("./file.xml") or die("error");
$start = $dom->documentElement;
fc($start);
function fc($node) {
 $child = $node->childNodes;
 foreach($child as $item) {
 if ($item->nodeType == XML_TEXT_NODE) {
 if (strlen(trim($item->nodeValue))) echo trim($item->nodeValue)."<br/>";
 }
 else if ($item->nodeType == XML_ELEMENT_NODE) fc($item);
 }
}
?>

After seeing many complaints about certain DOMDocument shortcomings, such as bad handling of encodings and always saving HTML fragments with <html>, <head>, and DOCTYPE, I decided that a better solution is needed.
So here it is: SmartDOMDocument. You can find it at http://beerpla.net/projects/smartdomdocument/
Currently, the main highlights are:
- SmartDOMDocument inherits from DOMDocument, so it's very easy to use - just declare an object of type SmartDOMDocument instead of DOMDocument and enjoy the new behavior on top of all existing functionality (see example below).
- saveHTMLExact() - DOMDocument has an extremely badly designed "feature" where if the HTML code you are loading does not contain <html> and <body> tags, it adds them automatically (yup, there are no flags to turn this behavior off).
Thus, when you call $doc->saveHTML(), your newly saved content now has <html><body> and DOCTYPE in it. Not very handy when trying to work with code fragments (XML has a similar problem).
SmartDOMDocument contains a new function called saveHTMLExact() which does exactly what you would want - it saves HTML without adding that extra garbage that DOMDocument does.
- encoding fix - DOMDocument notoriously doesn't handle encoding (at least UTF-8) correctly and garbles the output.
SmartDOMDocument tries to work around this problem by enhancing loadHTML() to deal with encoding correctly. This behavior is transparent to you - just use loadHTML() as you would normally.
- SmartDOMDocument Object As String - you can use a SmartDOMDocument object as a string which will print out its contents.
For example:
<?php
echo "Here is the HTML: $smart_dom_doc";
?>

I'm going to maintain this code and try to fix bugs as they come in.
Enjoy.

A simple function to grab all links in a page. 
 function get_links($url) {
 // Create a new DOM Document to hold our webpage structure 
 $xml = new DOMDocument();
 // Load the url's contents into the DOM 
 $xml->loadHTMLFile($url);
 // Empty array to hold all links to return 
 $links = array();
 //Loop through each <a> tag in the dom and add it to the link array 
 foreach ($xml->getElementsByTagName('a') as $link) {
 $url = $link->getAttribute('href');
 if (!empty($url)) {
 $links[] = $link->getAttribute('href');
 }
 }
 //Return the links 
 return $links;
 }

In this post http://softontherocks.blogspot.com/2014/11/descargar-el-contenido-de-una-url_11.html I found a simple way to get the content of a URL with DOMDocument, loadHTMLFile and saveHTML().
function getURLContent($url){
 $doc = new DOMDocument;
 $doc->preserveWhiteSpace = FALSE;
 @$doc->loadHTMLFile($url);
 return $doc->saveHTML();
}