Laravel Debugbar (Integrates PHP Debug Bar)
Clockwork - php dev tools in your browser - server-side component
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().
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.
The DebugBundle allows greater integration of the VarDumper component in the Symfony full-stack framework.
Xdebug — Step Debugger and Debugging Aid for PHP
The Debug component provides tools to ease debugging PHP code.