Travis-CI is a continuous integration tool that will run tests for a GitHub repository every time commits are pushed.
Matomo uses Travis to automatically run its test suite on every commit (for every branch and pull request). PHP and UI tests are run on the matomo-org/matomo build
Current status for 4.x-dev branch: 
Plugins can do the same if they include a .travis.yml file in their github repository. You can generate this file using the generate:travis-yml console command:
$ ./console generate:travis-yml --plugin=MyPlugin --php-versions="7.2,7.4" --distribution="bionic" --sudo-false --verbose
The command will automatically detect if you have PHP and/or UI tests in your plugin's Test/ directory and create a .travis.yml file that will run them. The tests will be run against both Matomo 4.x-dev branch and against the latest stable version.
The generate:travis-yml command will be changed over time as we modify the travis build process. The generated .travis.yml file will check if it is out-of-date from within travis and let you know by failing the build. In such a case you will have to re-run the command and commit the changes to get the build to run again.
You can control how the generated .travis.yml file behaves, by setting certain environment variables in your .travis.yml file.
These variables let you download other, test your plugin against a specific Matomo version and more.
Below is the list of all supported environment variables:
TEST_AGAINST_CORE
This variable can be set to minimum_required_piwik to test against the required Matomo version specified in your plugin's plugin.json manifest.
This variable should not be set as a global environment variable, instead it should be added as an entry in your .travis.yml file's matrix: section, eg:
env:
matrix:
- TEST_SUITE=PluginTests MYSQL_ADAPTER=PDO_MYSQL TEST_AGAINST_CORE=minimum_required_piwik
By default, one build will run the PHP tests against the minimum required Matomo version.
PIWIK_TEST_TARGET
This variable can be used to specify an exact Matomo version number / branch or the maximum_supported_piwik the test should run against. By specifying maximum_supported_piwik your tests will automatically run against the highest supported Matomo version number in your plugin.json. If no maximum Matomo version is specified, the tests will automatically run against the latest beta. To specify a max version in plugin.json you can eg specify "require": {"piwik": ">=2.15.0-rc1,<=2.15.1-b11"}. In this case max supported version is 2.15.1-b11. For more information on how to specify supported Matomo versions in your plugin have a look at the distributing your Matomo plugin guide.
This variable should be set as a global environment variable, eg:
env:
global:
- PIWIK_TEST_TARGET="maximum_supported_piwik" // run test against max supported version...
- PIWIK_TEST_TARGET="2.15.0" // ... or against a specific version
DEPENDENT_PLUGINS
This variable should be set to a space separated list of git repository slugs. Before running tests on travis, these repositories will be cloned. If your plugin depends on other plugins, you can use this variable to make sure your tests pass and/or test as much functionality as possible on travis.
This variable should be set as a global environment variable, eg:
env:
global:
- DEPENDENT_PLUGINS="myGithubAccount/myDependentPlugin myGithubAccount/myOtherDependentPlugin"
Should a dependent plugin be from a private repository, then you need to go Travis and select the build for your plugin (the plugin you are wanting to run the tests for, not the dependent plugin). Next go to "Settings". In the "Environment Variables" section add a new variable named "GITHUB_USER_TOKEN" and specify the token. Make sure to have the setting "DISPLAY VALUE IN BUILD LOG" disabled so the token value won't be visible to others. You can generate this github user token by going to https://github.com/settings/tokens and generating a new token. Make sure to select all checkboxes in the "repo" scope.
Plugins that use technologies other than MySQL or PHP may require extra setup and install steps to be executed on travis before running tests.
LoginLdap, for example, tests itself against a live LDAP server and thus needs to install and setup OpenLDAP on travis. To accomplish this, LoginLdap adds extra steps to its generated .travis.yml file.
To add extra steps to your plugin's .travis.yml file, create a /tests/travis folder inside your plugin and add one or more of the following special .yml files:
The contents of the XXX.before.yml files will be prepended to the specific section in your .travis.yml file, while the contents of the XXX.after.yml files will be appended.