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)
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.
Debug Bar Extension for the Timber Library
The Debug component provides tools to ease debugging PHP code.
Bugsnag notifier for the Laravel PHP framework. Monitor and report Laravel errors.
PHP logging library that is highly extendable and simple to use.