Xdebug and provides a range of features to improve the PHP development experience.
Step Debugging A way to step through your code in your IDE or editor while the script is executing.
Improvements to PHP's error reporting An improved var_dump() function, stack traces for Notices, Warnings, Errors and Exceptions to highlight the code path to the error
Tracing Writes every function call, with arguments and invocation location to disk. Optionally also includes every variable assignment and return value for each function.
Profiling Allows you, with the help of visualisation tools, to analyse the performance of your PHP application and find bottlenecks.
Code Coverage Analysis To show which parts of your code base are executed when running unit tests with PHP Unit.