Ajax Unit Tests for WordPress Plugin Development

The past couple of days I was having a hard time figuring out why my unit tests for AJAX were giving me an error stating Serialization of 'Closure' is not allowed. I’ve since fixed it and wanted to post about it because why not. For those WordPress plugin developers who use PHPUnit, let’s see if you spot the error. I’m new to PHPUnit, as well as unit-testing in general, and was not aware of this.

Things I installed:

• wp scaffold plugin testplugin
• composer require phpunit/phpunit ^9 --dev
• composer require yoast/phpunit-polyfills --dev

This is the actual test class I was trying, saved to a testplugin/tests/test-ajax.php file.

/**
 * Test cases for AJAX functions
 *
 * @group ajax
 * @runTestsInSeparateProcesses
 */
class AjaxTest extends WP_Ajax_UnitTestCase {

	public function setUp(): void
	{
		parent::setUp();
	}

	public function testIsTrue(): void
	{
		$this->assertTrue( true );
	}
}

And here is the output when running the test.

./vendor/bin/phpunit --group ajax --debug --verbose
Installing...
Running as single site... To run multisite, use -c tests/phpunit/multisite.xml
Not running ms-files tests. To execute these, use --group ms-files.
Not running external-http tests. To execute these, use --group external-http.
PHPUnit 9.5.24 #StandWithUkraine

Runtime:       PHP 8.0.22
Configuration: /var/www/test/wp-content/plugins/testplugin/phpunit.xml.dist

Serialization of 'Closure' is not allowed

---

I was following this article that mentions to use @runTestsInSeparateProcesses, because it “tells PHPUnit to run each test from this class in a separate process”.

I’ve figured out that this annotation is what was causing this error. I’m not sure if it was because of the environment in which I was testing? I use Windows Subsystem for Linux for development while at work. But I don’t know how this really would affect it. I assume that it should have ran in a new process within WSL. Let me know if you know.

Here is my original StackExchange question, which I answered myself. https://wordpress.stackexchange.com/questions/409190