GeSHi is designed to be a simple but powerful highlighting class, with the goal of supporting a wide range of popular languages. Developers can easily add new languages for highlighting and define easily customizable output formats.

MagickWand For PHP is a PHP module suite for working with the ImageMagick API, which lets you create, compose and edit bitmap images. It’s a useful tool for quickly incorporating image-editing features in your PHP applications.

pChart is a PHP class-oriented framework designed to create aliased charts. Most of today’s chart libraries have a cost; this one is free. Data can be retrieved from SQL queries or CSV files or can be manually provided.

Pixy is a Java program that performs automatic scans of PHP 4 source code, aimed to detect XSS and SQL injection vulnerabilities. Pixy takes a PHP program as input and creates a report that lists possible vulnerable points in the program, along with additional information for understanding the vulnerability.

MacGDBp is a live PHP debugger application for the Mac OS. It has all the features you’d expect from a fully featured debugger, such as the ability to step through your code and set breakpoints.

PHP_Dyn is another excellent PHP debugging tool that’s open-source. You can trace execution and get an output of the argument and return values of your functions.

PHP_Debug is an open-source project that gives you useful information about your PHP code that can be used for debugging. It can output processing times of your PHP and SQL, check the performance of particular code blocks and get variable dumps in graphical form, which is great if you need a more visual output than the one given to you by print_r() or var_dump().

DBG is a robust and popular PHP debugger for use in local and remote PHP debugging. It plugs into numerous PHP IDE’s and can easily be used with the command line.

As the name implies, Gubed PHP Debugger is a PHP debugging tool for hunting down logic errors.

Xdebug is one of the most popular debugging PHP extensions. It provides a ton of useful data to help you quickly find bugs in your source code. Xdebug plugs right into many of the most popular PHP applications, such as PHPEclipse and phpDesigner.