refactor: require PHP 7.1 minimum, add nullable types and return type hints in client tests

[armando-rodriguez-cko]

This pull request introduces significant updates to improve PHP version compatibility, modernize type safety in the codebase, and enhance CI workflows. The changes primarily focus on updating PHP dependencies, adding type hints, and restructuring workflows for better maintainability.

PHP Version Compatibility and Dependency Updates:

• Updated the minimum PHP version requirement in composer.json and README.md to >=7.1 and added guidance for resolving compatibility issues with PHP 8.1+ by upgrading dependencies like phpunit/phpunit and guzzlehttp/guzzle. [1] [2]
• Introduced conditional dependency installation in CI workflows to ensure compatibility with different PHP versions. [1] [2] [3]

Type Hinting and Code Modernization:

• Added strict type hints (e.g., string, ?string, array) across multiple methods in ApiClient.php, CheckoutUtils.php, and EventsClient.php to improve type safety and readability. [1] [2] [3]
• Refactored null checks and array handling to use modern PHP practices, such as strict comparisons and concise array initialization. [1] [2]

Continuous Integration (CI) Workflow Enhancements:

• Updated .github/workflows files to expand PHP version testing matrix, remove outdated PHP 5.6-specific logic, and streamline dependency management steps. [1] [2] [3]
• Added steps for generating and printing release versions in the build-release.yml workflow.

These changes ensure that the codebase is better aligned with modern PHP standards, improves compatibility with newer PHP versions, and enhances CI processes for better maintainability and reliability.

[Tobion]

PHP 7.1 is not supported anymore since a long time. Not even PHP 8.0 is anymore. So IMO the minimum version should be PHP 8.1 now.

[armando-rodriguez-cko]

@Tobion thank you for your comments, in new releases we will change the minimum version to 8.1