PHP Useful Extensions, Utilities and Classes

SimplePie
SimplePie is a PHP class that helps you work with RSS feeds. Check out the online RSS and Atom feed reader, which demonstrates a simple Web application that uses SimplePie.
HTML Purifier
HTML Purifier is a standards-compliant HTML filter library written in PHP. HTML Purifier not only removes all malicious code (better known as XSS) with a thoroughly audited, secure yet permissive white list, it also makes sure your documents are standards-compliant. Open source and highly customizable.
TCPDF
TCPDF is an open-source PHP class for generating PDF documents.
htmlSQL
htmlSQL is a unique tool. It is a PHP class for querying HTML values in an SQL-like syntax. Check out the live demonstration of how htmlSQL works.
The Greatest PHP Snippet File Ever (Using Quicktext for Notepad++)
“A little something for all coders: a snippets file that I use for PHP coding. This is designed to be used with Quicktext for Notepad++, but feel free to adapt it to whatever text editor you prefer.”
Creole
Creole is a database abstraction layer for PHP5. It abstracts PHP’s native database-specific API to create more portable code while also providing developers with a clean, fully object-oriented interface based loosely on the API for Java’s JDBC.
PHPLinq
LINQ is a component that adds native data querying capabilities to PHP using a syntax reminiscent of SQL. It defines a set of query operators that can be used to query, project and filter data in arrays, enumerable classes, XML, relational databases and third-party data sources. [via]
PHPMathPublisher
With PhpMathPublisher, you can publish mathematical documents on the Web using only a PHP script (no LaTeX programs on the server and no MathML).
phpMyAdmin
If you’re working with PHP, there’s a big chance you’re set up in a LAMP configuration. phpMyAdmin is Web-based tool for managing, building, importing, exporting and exploring MySQL databases.
PHPExcel
PHPExcel is a set of useful PHP classes for working with Microsoft Excel files. PHPExcel allows you to read Excel files and write to them. This is useful for dynamically generating Excel spreadsheets for downloading.
Phormer
Phormer is a PHP-based photo gallery management application that helps you to store, categorize and trim your photos online.
xajax PHP Class Library
xajax is a PHP class for easily working with PHP AJAX applications. It gives you an easy-to-use API for quickly managing AJAX-related tasks. Check out the xajax Multiplier demo and the Graffiti Wall demo to see the xajax PHP class in action.
PHP User Class
PHP User Class is an excellent script that helps you create a system for user authentication (i.e. registration, log in, account profile, etc.). It’s a useful utility to have around if you require user registration for your Web applications.
PHP-GTK
PHP-GTK is a PHP extension for the GTK+ toolkit (a robust toolkit for developing GUIs). It is a suite of useful OOP functions and classes to help you rapidly build cross-platform, client-side GUI’s for your application.