Add PHP linter and PHP linting in GitHub Actions

[eason9487]

Changes proposed in this Pull Request:

Implements parts of #4

• Add PHP linter and PHP linting in GitHub Actions
• Fix all linting errors.

Detailed test instructions:

1. Run composer install
2. Run npm run lint:php locally to test the linter
3. View the failed run that was intentionally left with some linting errors
4. View the error and warning annotations with this link
5. View the successful run
6. View automatically fixed errors with this diff
7. View manually fixed errors with this diff