WordPress Plugin Development using TDD, Docker and WpUnitTestCase

If you landed in this article is because you already are aware that testing your projects is important.

Also, you might already be aware that testing in WordPress is hard, very hard, since it has hundreds upon hundreds of functions that you need to Mock to make your code work.

Finally, you might also be ware that setting up testing in a project that uses a databases is something that can be complex for the amount of non portable configuration it involves.

Well, you are in the right place, because here I’m going to show you how to configure testing in you WordPress plug-in using nothing but docker so you can distribute your environment with your team or move it to a new machine easily.



Even though you the final environment is pretty portable, you still need to install a couple of pieces of software, being the more obvious Docker

Optionally, you can install composer in your development machine. This is optional since we are going to install composer in our Docker container, but having a local instance will speed up your development process. Specially if you want to execute phpcs or phpcbf (wich we won’t touch in this article) against your code.

Before we continue, make sure you have access to the docker-compose command. Unless you have a very old version of Docker, you must have access to it.

Terminal showing versions of docker, docker-compose and composer

Create a plugin

For this example we’re going to test a very simple WordPress plugin. And the beauty of our current approach, is that you can create this plug-in wherever you want. You don’t need a WordPress installation nor a database or an Apache server.

So create an empty directory…

mkdir wordpress-tdd-plugin
cd $_
git init
touch .gitignore
git add .
git commit -m "First commit"
git branch -m main

And then create the plug-in file itself.

We are going to do a very simple plug-in that provides the tdd-shortcode shortcode making sure that it uses some native WordPress functions. For that we just need to create the file wordpress-tdd-pluging.php in our directory with the following content:

 * Plugin Name:       WordPress TDD Plugin
 * Plugin URI:        https://marioyepes.com
 * Description:       Example plugin on how to use TDD in WordPress
 * Version:           1.0.0
 * Requires at least: 4.0
 * Requires PHP:      7.2
 * Author:            Mario Yepes
 * Author URI:        https://marioyepes.com
 * License:           GPL v2 or later
 * License URI:       https://www.gnu.org/licenses/gpl-2.0.html
 * Update URI:        https://example.com/my-plugin/
 * Text Domain:       wp-tdd
 * Domain Path:       /languages

function create_shorcode_tdd_plugin( $atts, $content = '' ) {

        $default_font  = get_option( 'tdd_font', 'Arial, Helvetica, sans-serif');
        $default_color = get_option( 'tdd_color', '');
        $atts          = shortcode_atts(array(
                'font' => $default_font,
                'color' => $default_color,
                'weight' => 'normal'
        ), $atts);

        return '<div style="font-family: ' . $atts['font'] .  '; color: ' . $atts['color'] .
                '; font-weight: ' .  $atts['weight'] . '">' . $content . '</div>';

add_shortcode( 'tdd-shortcode', 'create_shorcode_tdd_plugin' );

Right from the start you can see that this plug-in is going to be somewhat difficult to test since it uses the get_option and add_shortcode WordPress functions. But that’s something we’re going to solve in the following steps.

Creating the Application Server

And by Application Server I mean the Docker Container that will run Apache and PHP (not MySQL). And for that we’ll create our first Docker image using the following Dockerfile

# Dockerfile
FROM wordpress

ARG PLUGIN_NAME=wordpres-tdd-plugin

# Setup the OS
RUN apt-get -qq update ; apt-get -y install unzip curl sudo subversion mariadb-client \
        && apt-get autoclean \
        && chsh -s /bin/bash www-data

# Install wp-cli
RUN curl https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar > /usr/local/bin/wp-cli.phar \
        && echo "#!/bin/bash" > /usr/local/bin/wp-cli \
        && echo "su www-data -c \"/usr/local/bin/wp-cli.phar --path=/var/www/html \$*\"" >> /usr/local/bin/wp-cli \
        && chmod 755 /usr/local/bin/wp-cli* \
        && echo "*** wp-cli command installed"

# Install composer
RUN php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');" \
        && php composer-setup.php \
        && php -r "unlink('composer-setup.php');" \
        && mv composer.phar /usr/local/bin/ \
        && echo "#!/bin/bash" > /usr/local/bin/composer \
        && echo "su www-data -c \"/usr/local/bin/composer.phar --working-dir=/var/www/html/wp-content/plugins/${PLUGIN_NAME} \$*\"" >> /usr/local/bin/composer \
        && chmod ugo+x /usr/local/bin/composer \
        && echo "*** composer command installed"

This will create a “Virtual Machine” (Is not a virtual machine but it helps to think of it as one) based in the official Docker WordPress image with 3 modifications:

To convert this file to something we can run, we need to create an image. This is achieved with the following command:

docker build -t wp-tdd-image .
docker images # to verify it's creation

Note that I’m building the image with the wp-tdd-image tag.

First docker image creation

This creates the Docker Image, but it doesn’t run it. For that we need to execute it:

docker run --rm -p 8080:80 wp-tdd-image

And to verify that is actually running, visit http://localhost:8080. Do not run the installer. It will fail since we still haven’t provided a database.

WordPress installer in the browser

Now that we know that the Dockerfile works, we can remove the image with

docker image rmi wp-tdd-image

Adding a database

Instead of adding a database to our current image (aka current virtual machine), we’re going to use an already created image. But for that to work we have to use docker-compose which requires the new file docker-compose.yml:

# docker-compose.yml
version: "3"

  # MySQL database
    image: mariadb
    restart: unless-stopped
    container_name: tdd-plugin-db
      MYSQL_DATABASE: wordpress
      MYSQL_USER: wordpress
      MYSQL_PASSWORD: wordpress
      - db-data:/var/lib/mysql

  # Wordpress
      context: .
      dockerfile: Dockerfile
    restart: unless-stopped
    container_name: tdd-plugin-app
      WORDPRESS_DB_HOST: tdd-plugin-db
      WORDPRESS_DB_USER: wordpress
      WORDPRESS_DB_PASSWORD: wordpress
      WORDPRESS_DB_NAME: wordpress
      - wp-data:/var/www/html
      - ./:/var/www/html/wp-content/plugins/wordpress-tdd-plugin
      - 8080:80
      - db

# Make network name pretty
    name: tdd-plugin-network

# Persist DB and WordPress data across containers

Let’s explain a little this docker-compose.yml. First the part of the MySQL image:

Now, for the WordPress image, I’m telling docker-compose to use the Dockerfile and:

Finally, let’s install WordPress by starting this environment a visiting http://localhost:8080 to get the WordPress install wizard

docker-compose up
# Open your browser and go to http://localhost:8080

Wordpress Install Wizard

Now you can install WordPress and verify that the plug-in is present and valid by visiting your http://localhost:8080/wp-admin/plugins.php page:

The WordPress TDD Plugin in the plugin list

This is why I love Docker… It saves me the trouble of installing those tools in my machine. Also, it guards me from running into compatibility issues since every development environment is completely isolated from any other project you have.

Setting up testing

This is the challenging part. Not because is hard, but because it has a couple of tricks.

If you read carefully the Dockerfile we used to setup wordpress, you might have noticed that we installed wp-cli and composer inside the image. The reason for that is so we can:

So, open another terminal (since the docker-compose is using the one you previously had) and execute the following:

docker-compose exec wp wp-cli scaffold plugin-tests wordpress-tdd-plugin

wordpres-tdd-plugin is the name of our current plug-in

This command is supposed to scaffold, or initialize, the testing files in your plug-in directory. The docker-compose exec wp part of the command is to have it be executed inside the current running container.

Scaffold fails

It fails… The reason being that the Docker container is trying to execute the command as the www-data user and your local machine either does not have that user, or that users exists but doesn’t have write access to your directory.

To solve this issue, we have to do our first trick:

Creating scaffold placeholder files

This time it works! And now we should have the following structure:

$ tree .
├── Dockerfile
├── bin
│   └── install-wp-tests.sh
├── docker-compose.yml
├── phpunit.xml.dist
├── tests
│   ├── bootstrap.php
│   └── test-sample.php
└── wordpress-tdd-plugin.php

2 directories, 7 files

We still don’t have tests, or even WordPress mock functions. But at least we have the means to create them.

So, What did we do????

What the docker-compose exec wp wp-cli scaffold plugin-tests wordpress-tdd-plugin command does is:

Upgrade the WordPress image

Believe it or not, we are not ready to start testing!

We have all the required files in place, but we still have to create a WordPress Testing Environment in our docker container. And for that we need to update our Dockerfile:

# Dockerfile
FROM wordpress

ARG PLUGIN_NAME=wordpres-tdd-plugin

# Setup the OS
RUN apt-get -qq update ; apt-get -y install unzip curl sudo subversion mariadb-client \
        && apt-get autoclean \
        && chsh -s /bin/bash www-data

# Install wp-cli
RUN curl https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar > /usr/local/bin/wp-cli.phar \
        && echo "#!/bin/bash" > /usr/local/bin/wp-cli \
        && echo "su www-data -c \"/usr/local/bin/wp-cli.phar --path=/var/www/html \$*\"" >> /usr/local/bin/wp-cli \
        && chmod 755 /usr/local/bin/wp-cli* \
        && echo "*** wp-cli command installed"

# Install composer
RUN php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');" \
        && php composer-setup.php \
        && php -r "unlink('composer-setup.php');" \
        && mv composer.phar /usr/local/bin/ \
        && echo "#!/bin/bash" > /usr/local/bin/composer \
        && echo "su www-data -c \"/usr/local/bin/composer.phar --working-dir=/var/www/html/wp-content/plugins/${PLUGIN_NAME} \$*\"" >> /usr/local/bin/composer \
        && chmod ugo+x /usr/local/bin/composer \
        && echo "*** composer command installed"

# Create testing environment
COPY --chmod=755 bin/install-wp-tests.sh /usr/local/bin/
RUN echo "#!/bin/bash" > /usr/local/bin/install-wp-tests \
        && echo "su www-data -c \"install-wp-tests.sh \${WORDPRESS_DB_NAME}_test root root \${WORDPRESS_DB_HOST} latest\"" >> /usr/local/bin/install-wp-tests \
        && chmod ugo+x /usr/local/bin/install-wp-test* \
        && su www-data -c "/usr/local/bin/install-wp-tests.sh ${WORDPRESS_DB_NAME}_test root root '' latest true" \
        && echo "*** install-wp-tests installed"

The changes are at the bottom, and we’re basically telling the wp image to copy the bin/install-wp-tests.sh files inside the container and execute that command upon container creation.

So, stop the containers by typing Ctrl-c in the terminal, and the rebuild the wp image with:

# Ctr-C on the terminal where docker is running
docker-compose build wp
docker-compose up

Install the WordPress test files and PHPUnit

In the last step we added the install-wp-tests command to our container. Now we have to execute it so it re-creates the WordPress testing files but with access to our database:

docker-compose exec wp install-wp-tests 

What this does is to create a “mock” WordPress inside the container in the path /tmp/wordpress. This will enable us to make tests without the need to mock get_option and add_shortcode, or any other WordPress function, ourselves.

Then we need to install PHPUnit in our plug-in directory but using the same trick of creating empty files:

touch composer.json composer.lock
mkdir vendor
chmod 777 vendor composer.json composer.lock
docker-compose exec wp composer require phpunit/phpunit=^7
docker-compose exec wp install-wp-tests

Notice that we are using PHPUnit version 7 since that is the current version that WordPress supports.

Executing the sample-tests.php

We’re almost done with the setup. We just need to add a new scripts in our composer.json file so we can run our tests easily:

  "require": {
    "phpunit/phpunit": "^7"
  "scripts": {
    "phpunit": "phpunit"

And edit the phpunit.xml.dist file, enabling the sample test:

<?xml version="1.0"?>
            <testsuite name="sample">
                    <directory prefix="test-" suffix=".php">./tests/</directory>
                    <!-- <exclude>./tests/test-samplejG.php</exclude> -->

Finally, execute the tests trough docker-compose:

docker-compose exec wp composer phpunit

And we’re done with the setup. Now let’s actually verify that we can test our plugin

Our first custom test

At the beginning of this article, we created a simple plug-in with a custom function that we wanted to test. We’re finally ready to verify that function by creating a test.

So create the tests/test-tdd-shortcode.php file with a custom test:

The name of the file has to start with test-

 * Class TddTest
 * @package Wordpress_Tdd_Plugin

 * Wordpress TDD plugin tests
class TddTest extends WP_UnitTestCase {

     * Test shortcode output
    public function test_tdd_shortcode() {
        add_option('tdd_font', 'monospaced');
        add_option('tdd_color', 'red');

        $actual = create_shorcode_tdd_plugin(array(), 'hola');
        $expected = '<div style="font-family: monospaced; color: red; font-weight: normal">hola</div>';

        $this->assertEquals($actual, $expected);
        $this->assertTrue(false); // Just to verify phpunit

Notice that we introduced an error. The last line of the function is a false assertion that will help us to verify that the tests are actually being run.

Les verify that by executing

docker-compose exec wp composer phpunit

Running our custom test with a false assertion

And even tough that last assertion fails, we are certain that our tests are being run.

Also notice that we used the add_option function. That’s a WordPress function. So we are certain that our tests are using the WordPress test suite.


We’re done for this article. But if you want to dig deeper into WordPress testing I recommend reading the following blog posts

And if you want to take a look at the generated files for this article, you can visit the GitHub repo