After talking about how to write tests, let's stop and talk about what to test.
Don't Write Tests. You Don't Have Time. Right? Right?!..
The number one reason people don't write tests is time. They feel like they must write many tests, and it takes a lot of time to reach good code coverage.
People don't have enough time to write tests, which is ironic because they need to spend more time with each new feature testing, manually debugging, and fixing bugs that automated tests would otherwise catch.
In the long run, the argument "We don't have time to write tests" makes no logical sense.
The exception may be if your project is personal, with almost no users and only a few new features upcoming. Or, if you want to take risks and let your customers perform all the testing for you and report the bugs so you can fix them.
Side note: customers usually don't report bugs. They just leave bad reviews and move on to using other software in the future.
With that in mind, I assume that if you are reading this course, you want to do testing, and your goal is to write as many tests as possible. But hold on, it's not the goal.
Code/Test Coverage
Historically, this "code coverage" thing was really important and was emphasized by everyone that you need to achieve 100% code/test coverage. But let's look at it from a realistic perspective.
So, what is code coverage? Theoretical Wikipedia definition is "a percentage of which source code is executed when the test suite is run". What does it mean in practice?
The first example is from an article Generate Code Coverage Report in Laravel and make it 100% score easily. In this article, the author generates a code report in Laravel with Xdebug. So, you launch the test with test coverage, and it shows which of those methods were executed during the test being run.
The second example is passing the --coverage option to the php artisan test command. Before passing this option, the Xdebug should be configured. Then, after all the tests, you will see an output of your code coverage.
As you see, some files are covered 100%, and some are not. Now, do you want to achieve...