PHPUnit Polyfills is a PHP library that provides polyfill traits for PHPUnit assertion methods that were introduced, renamed, or removed across PHPUnit versions 7.5 through 12.x. It allows developers to write tests using the latest PHPUnit 12.x assertion API while still running them on older PHPUnit versions. The library offers polyfill traits for assertions like assertObjectEquals, assertIsList, assertContainsOnlyBool, expectUserDeprecationMessage, and many more. It also provides two TestCase base classes to handle fixture method signature mismatches introduced in PHPUnit 8.0. Installation is via Composer, and it supports PHP 7.1+.