Xdebug is a PHP extension that provides the debugging, profiling, and tracing facilities that the language itself does not. It is loaded as a Zend extension, hooks into the PHP engine, and then speaks the DBGp protocol over a TCP socket to an IDE like PhpStorm, VS Code, or Vim, providing step debugging, breakpoints, watches, and stack inspection for running scripts and web requests.
Beyond the debugger, Xdebug has several distinct modes that can be enabled independently. develop replaces PHP’s plain var_dump output with a prettified, color-aware version and improves error messages with full stack traces. profile writes Cachegrind-format files that KCachegrind or QCachegrind can visualize to find hot paths. trace writes a line-by-line execution log of every function call, arguments, and return values. coverage is the engine PHPUnit and other test runners use to produce code coverage reports.
It has been maintained by Derick Rethans since 2002 and is the de facto debugger for PHP, typically enabled only in development and CI environments because of its runtime cost.