PHPUnit Manual

Even good programmers make mistakes. The difference between a good programmer and a bad programmer is that the good programmer uses tests to detect his mistakes as soon as possible. The sooner you test for a mistake the greater your chance of finding it and the less it will cost to find and fix. This explains why leaving testing until just before releasing software is so problematic. Most errors do not get caught at all, and the cost of fixing the ones you do catch is so high that you have to perform triage with the errors because you just cannot afford to fix them all.

Testing with PHPUnit is not a totally different activity from what you should already be doing. It is just a different way of doing it. The difference is between testing, that is, checking that your program behaves as expected, and performing a battery of tests, runnable code-fragments that automatically test the correctness of parts (units) of the software. These runnable code-fragments are called unit tests.

In this chapter we will go from simple print-based testing code to a fully automated test. Imagine that we have been asked to test PHP's built-in array. One bit of functionality to test is the function count(). For a newly created array we expect the count() function to return 0. After we add an element, count() should return 1. Example 1.1 shows what we want to test.

<?php
$fixture = array();
// $fixture is expected to be empty.
 
$fixture[] = 'element';
// $fixture is expected to contain one element.
?>

A really simple way to check whether we are getting the results we expect is to print the result of count() before and after adding the element (see Example 1.2). If we get 0 and then 1, array and count() behave as expected.

<?php
$fixture = array();
print count($fixture) . "\n";
 
$fixture[] = 'element';
print count($fixture) . "\n";
?>

0
1

Now, we would like to move from tests that require manual interpretation to tests that can run automatically. In Example 1.3, we write the comparison of the expected and actual values into the test code and print ok if the values are equal. If we ever see a not ok message, we know something is wrong.

<?php
$fixture = array();
print count($fixture) == 0 ? "ok\n" : "not ok\n";
 
$fixture[] = 'element';
print count($fixture) == 1 ? "ok\n" : "not ok\n";
?>

ok
ok

We now factor out the comparison of expected and actual values into a function that raises an Exception when there is a discrepancy (Example 1.4). This gives us two benefits: the writing of tests becomes easier and we only get output when something is wrong.

<?php
$fixture = array();
assertTrue(count($fixture) == 0);
 
$fixture[] = 'element';
assertTrue(count($fixture) == 1);
 
function assertTrue($condition)
{
    if (!$condition) {
        throw new Exception('Assertion failed.');
    }
}
?>

The test is now completely automated. Instead of just testing as we did with our first version, with this version we have an automated test.

The goal of using automated tests is to make fewer mistakes. While your code will still not be perfect, even with excellent tests, you will likely see a dramatic reduction in defects once you start automating tests. Automated tests give you justified confidence in your code. You can use this confidence to take more daring leaps in design (Refactoring), get along with your teammates better (Cross-Team Tests), improve relations with your customers, and go home every night with proof that the system is better now than it was this morning because of your efforts.