How do you parse and process HTML/XML in PHP?

Native XML Extensions

I prefer using one of the native XML extensionssince they come bundled with PHP, are usually faster than all the 3rd party libs and give me all the control I need over the markup.

DOM

The DOM extension allows you to operate on XML documents through the DOM API with PHP 5. It is an implementation of the W3C's Document Object Model Core Level 3, a platform- and language-neutral interface that allows programs and scripts to dynamically access and update the content, structure and style of documents.

DOM is capable of parsing and modifying real world (broken) HTML and it can do XPath queries. It is based on libxml.
It takes some time to get productive with DOM, but that time is well worth it IMO. Since DOM is a language-agnostic interface, you'll find implementations in many languages, so if you need to change your programming language, chances are you will already know how to use that language's DOM API then.
A basic usage example can be found in Grabbing the href attribute of an A element and a general conceptual overview can be found at DOMDocument in php
How to use the DOM extension has been covered extensively on StackOverflow, so if you choose to use it, you can be sure most of the issues you run into can be solved by searching/browsing Stack Overflow.

XMLReader

The XMLReader extension is an XML pull parser. The reader acts as a cursor going forward on the document stream and stopping at each node on the way.

XMLReader, like DOM, is based on libxml. I am not aware of how to trigger the HTML Parser Module, so chances are using XMLReader for parsing broken HTML might be less robust than using DOM where you can explicitly tell it to use libxml's HTML Parser Module.
A basic usage example can be found at getting all values from h1 tags using php

SimpleXml

The SimpleXML extension provides a very simple and easily usable toolset to convert XML to an object that can be processed with normal property selectors and array iterators.

SimpleXML is an option when you know the HTML is valid XHTML. If you need to parse broken HTML, don't even consider SimpleXml because it will choke.
A basic usage example can be found at A simple program to CRUD node and node values of xml file and there is lots of additional examples in the PHP Manual.

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3rd Party Libraries (libxml based)

If you prefer to use a 3rd-party lib, I'd suggest using a lib that actually uses DOM/libxml underneath instead of string parsing.

phpQuery

phpQuery is a server-side, chainable, CSS3 selector driven Document Object Model (DOM) API based on jQuery JavaScript Library written in PHP5 and provides additional Command Line Interface (CLI).

Zend_Dom

Zend_Dom provides tools for working with DOM documents and structures. Currently, we offer Zend_Dom_Query, which provides a unified interface for querying DOM documents utilizing both XPath and CSS selectors.

QueryPath

QueryPath is a PHP library for manipulating XML and HTML. It is designed to work not only with local files, but also with web services and database resources. It implements much of the jQuery interface (including CSS-style selectors), but it is heavily tuned for server-side use. Can be installed via Composer.

FluentDom

FluentDOM provides a jQuery-like fluent XML interface for the DOMDocument in PHP. Selectors are written in XPath, rather than CSS (claims to improve performance). Can be installed via Composer.

fDOMDocument

fDOMDocument extends the standard DOM to use exceptions at all occasions of errors instead of PHP warnings or notices. They also add various custom methods and shortcuts for convenience and to simplify the usage of DOM.

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3rd-Party (not libxml-based)

The benefit of building upon DOM/libxml is that you get good performance out of the box because you are based on a native extension. However, not all 3rd-party libs go down this route. Some of them listed below

SimpleHtmlDom

• An HTML DOM parser written in PHP5+ lets you manipulate HTML in a very easy way!
• Require PHP 5+.
• Supports invalid HTML.
• Find tags on an HTML page with selectors just like jQuery.
• Extract contents from HTML in a single line.

I generally do not recommend this parser. The codebase is horrible and the parser itself is rather slow and memory hungry. Any of the libxml based libraries should outperform this easily.

Ganon

• A universal tokenizer and HTML/XML/RSS DOM Parser
  • Ability to manipulate elements and their attributes
  • Supports invalid HTML and UTF8
• Can perform advanced CSS3-like queries on elements (like jQuery -- namespaces supported)
• A HTML beautifier (like HTML Tidy)
  • Minify CSS and Javascript
  • Sort attributes, change character case, correct indentation, etc.
• Extensible
  • Parsing documents using callbacks based on current character/token
  • Operations separated in smaller functions for easy overriding
• Fast and Easy

Never used it. Can't tell if it's any good.

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HTML 5

You can use the above for parsing HTML5, but there can be quirks due to the markup HTML5 allows. So for HTML5 you want to consider using a dedicated parser, like
html5lib

A Python and PHP 

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