PHPUnit is based on of the idea that developers should be able to find mistakes in their newly-committed code quickly and assert that no code regression has occurred in other parts of the code base. Much like other unit testing frameworks, PHPUnit uses assertions to verify that the behavior of the specific component - or "unit" - being tested behaves as expected.
The goal of unit testing is to isolate each part of the program and show that the individual parts are correct. A unit test provides a strict, written contract that the piece of code must satisfy. As a result, unit tests find problems early in the development cycle.