The next type of tests that we can generate are called functional tests. Functional tests allow us to emulate our application without running it through a web server. This provides us with a way to quickly test the output of our application without introducing the overhead of a web server.

This emulation process is achieved by directly manipulating the $_REQUEST, $_POST, and $_GET parameters before executing our application. As a side effect of this behavior, however, certain variables, such as $_SESSION and $_COOKIE, as well as headers, can result in junk errors being thrown, which wouldn't necessarily be thrown in a real environment. Moreover, with Codeception, our functional tests will be executing within a single memory container, which may result in a test failing when run as part of a group as opposed to running a single test. Additionally, unlike acceptance testing, functional testing can't emulate JavaScript and Ajax requests.

Overall, functional testing provides...