PHPUnit is a programmer-oriented testing framework for PHP.
It is an instance of the xUnit architecture for unit testing frameworks.
PHPUnit 5.7 is the old stable release series.
It became stable on December 2, 2016.
Support for PHPUnit 5 ends on February 2, 2018.
You can find out what was new in PHPUnit 5.7 in the release announcement.
PHPUnit 5.7 is supported on PHP 5.6, PHP 7.0, and PHP 7.1.
The documentation has details on how to verify PHAR releases of PHPUnit using PGP signatures and SHA1 hashes as well as how to install PHPUnit using Composer.
Old PHAR releases are archived at phar.phpunit.de.