WordPress Unit Testing: Your Safety Net for Complex Code

I got a call from a client running a decent-sized WooCommerce store. They needed a “quick change” to a custom coupon function. Seemed simple enough. An hour of work, tops. I pushed the change live, and everything looked good. For about ten minutes. Then the support tickets started rolling in. Subscription renewals were failing. Bulk discounts weren’t applying in the cart. A total mess.

That sinking feeling. We’ve all been there. A tiny change over here causes a five-alarm fire over there. This is precisely the kind of chaos that proper WordPress unit testing is meant to prevent. It’s the safety net that catches you before you deploy a nightmare.

Integration vs. Unit Tests: What’s the Difference?

Now, a lot of people hear “testing” and think about checking if a WordPress hook fires correctly or if `update_post_meta` saved the right value. That’s an integration test—you’re testing how your code *integrates* with the rest of WordPress. It’s important, but it’s slow, and it’s not what we’re talking about here.

A unit test is different. It’s about isolating a single piece of your code—one function, one “unit”—and testing its logic completely on its own. You don’t need to load WordPress. You don’t need a database. You just need to check if your function, given a specific input, produces the expected output. Trust me on this, it’s a faster and more precise way to validate your logic.

The Magic of Mocks

My mistake with that coupon function was that it called `get_option()` to fetch a tax rate. My “fix” worked, but I didn’t account for how renewals used that same function in a context where the option wasn’t loaded yet. Had I written a unit test, I would have caught it immediately. Here’s the kicker: you can do this by “mocking” the WordPress function.

A mock is a fake version of a function that you control. You tell it, “When you’re called with *this* argument, I want you to return *that* value.” This lets you test your function’s logic without touching the database or any other part of WordPress. It’s incredibly powerful. For a deeper dive into the technical setup, a post I read on carlalexander.ca years ago is still one of the best resources.

<?php
// The function we want to test
namespace MyPlugin;

function get_prefixed_option($name) {
    return get_option('my_plugin_' . $name);
}

// The unit test for our function
class MyPluginTest extends \PHPUnit\Framework\TestCase {
    use \phpmock\phpunit\PHPMock;

    public function test_get_prefixed_option() {
        // 1. Create a "mock" of WordPress's get_option()
        $get_option = $this->getFunctionMock('MyPlugin', 'get_option');

        // 2. Set expectations: we expect it to be called once
        // with 'my_plugin_api_key' and told it to return 'ABC-123'.
        $get_option->expects($this->once())
                   ->with($this->equalTo('my_plugin_api_key'))
                   ->willReturn('ABC-123');

        // 3. Run our function and assert the result is what we expect.
        $this->assertEquals('ABC-123', get_prefixed_option('api_key'));
    }
}

So, What’s the Point?

Writing tests like this feels like extra work at first. But it’s not about hitting some arbitrary “100% code coverage” metric. It’s about building a safety net that gives you the confidence to refactor, improve, and add features without breaking things. It forces you to write cleaner, more modular code by its very nature.

• It prevents simple bugs from becoming client-facing disasters.
• It lets you refactor old code without fear.
• It serves as documentation for what your code is supposed to do.

Look, this stuff gets complicated fast. If you’re tired of debugging someone else’s mess and just want your site to work, drop my team a line. We’ve probably seen it before.