Clockwork - php dev tools in your browser - server-side component
A REPL for PHP
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().
Bugsnag notifier for the Laravel PHP framework. Monitor and report Laravel errors.
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.
Xdebug — Step Debugger and Debugging Aid for PHP
Kint - a powerful and modern PHP debugging tool.
Debug Extension for Yii 2
SilverStripe DebugBar module