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().
Laravel Debugbar (Integrates PHP Debug Bar)
Clockwork - php dev tools in your browser - server-side component
Kint - a powerful and modern PHP debugging tool.
WordPress Plugin for debugging and learning with and at the application.
Xdebug — Step Debugger and Debugging Aid for PHP
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.
PHP errors for cool kids
The Debug component provides tools to ease debugging PHP code.