sebastianbergmann/phpunit

PHPUnit is the standard testing framework for PHP applications. Its purpose is to let developers write automated tests that verify their code works correctly, running checks on individual functions or components in isolation to catch bugs before they reach production. This style of testing, where you check small pieces of code independently, is called unit testing, and PHPUnit is the most widely adopted tool for doing this in the PHP ecosystem. The framework follows the xUnit architecture, a design pattern shared by testing frameworks across many languages (JUnit for Java, pytest for Python, etc.). A developer writes test classes that call their application code and make assertions, statements like "this function should return 5 when given these inputs." If an assertion fails, PHPUnit reports it clearly so the developer knows exactly what broke. PHPUnit also includes a code coverage tool that shows which parts of the codebase are actually exercised by tests, helping identify gaps. Installation is straightforward: you can download a single bundled file (called a PHAR) or add it as a dependency via Composer, which is PHP's standard package manager. The framework comes with a large ecosystem of companion packages covering areas like code coverage measurement, file iteration, and timing utilities, all maintained by the same author. You would use PHPUnit on any PHP project where you want confidence that changes do not break existing functionality, from small open source libraries to large commercial web applications. It is a foundational tool for PHP development teams practicing test-driven development or continuous integration.