SilverStripe DebugBar module
Clockwork - php dev tools in your browser - server-side component
WordPress Plugin for debugging and learning with and at the application.
Laravel Debugbar (Integrates PHP Debug Bar)
PHP errors for cool kids
The VarDumper component provides mechanisms for walking through any arbitrary PHP variable. It provides a better dump() function that you can use instead of var_dump().
Kint - a powerful and modern PHP debugging tool.
Tracy: the addictive tool to ease debugging PHP code for cool developers. Friendly design, logging, profiler, advanced features like debugging AJAX calls or CLI support. You will love it.