wp-phpunit-writing-tests

Writing WordPress PHPUnit tests

Once the harness is in place (wp-phpunit-test-setup), this skill is about the tests themselves: which base class, how to build fixtures, how to mock, and the integration-vs-unit decision that trips most people up.

When to use this skill

• Writing the first real tests for a plugin/theme.
• Reviewing tests for correct fixtures, isolation, and mocking.
• Deciding between an integration test and a true unit test.
• Mocking outbound HTTP (wp_remote_*) or WP functions.

Integration vs unit — get this right first

WP_UnitTestCase boots the full WordPress environment and uses a real test database. Its teardown and factory cleanup isolate normal WordPress records, but do not describe the suite as a universal per-test transaction: code that commits, creates tables/files, changes external services, starts cron workers, or mutates non-database globals still needs explicit cleanup. Despite the name, this is an integration test, not a pure unit test.

For true unit tests — fast, isolated, no WP boot, no DB — you mock WordPress functions with one of:

• Brain Monkey (brain/monkey, 2.7.x) — Mockery + Patchwork based; expressive when() / expect().
• WP_Mock (10up/wp_mock, 1.1.x) — explicit per-function expectations, action/filter helpers.

Rule of thumb: test your own logic (a calculator, a formatter, a class method) as a unit test with mocked WP calls; test interaction with WordPress (it really saved the post, the hook really ran) as a WP_UnitTestCase integration test. Most plugins want both, in separate test suites.

Integration tests: WP_UnitTestCase

Fixtures are snake_case — and that matters

class Test_My_Plugin extends WP_UnitTestCase {

    public function set_up(): void {
        parent::set_up();          // MUST be the first line
        // per-test arrange
    }

    public function tear_down(): void {
        // per-test cleanup
        parent::tear_down();       // MUST be the last line
    }

    public function test_it_does_a_thing(): void {
        $this->assertTrue( my_plugin_does_a_thing() );
    }
}