NOTE: If you didn't create a .php-cs-fixer.dist.php file, do that first before adding this workflow. An example of this file created by the creators of PHP-CS-Fixer can be found here.

You can use it as a GitHub Action like this:

# .github/workflows/lint.yml

on: [push, pull_request]
name: Main
jobs:
  php-cs-fixer:
    name: PHP-CS-Fixer
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
    - name: PHP-CS-Fixer
      uses: docker://oskarstark/php-cs-fixer-ga

To use a custom config, e.g. --diff and --dry-run options:

# .github/workflows/lint.yml

on: [push, pull_request]
name: Main
jobs:
  php-cs-fixer:
    name: PHP-CS-Fixer
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
    - name: PHP-CS-Fixer
      uses: docker://oskarstark/php-cs-fixer-ga
+     with:
+       args: --config=.project.php_cs --diff --dry-run

You can copy/paste the .github/ folder (under examples/) to your project and that's all!

A Docker image is built automatically and located here: https://hub.docker.com/r/oskarstark/php-cs-fixer-ga

You can run it in any given directory like this:

docker run --rm -it -w=/app -v ${PWD}:/app oskarstark/php-cs-fixer-ga:latest

You can find a working and not working PR here: https://github.com/OskarStark/test-php-cs-fixer-ga/pulls