Add PHPStan for static analysis and add support for PHP 7.4+

[Greg0]

Based on #5

Overview

This PR introduces PHPStan static analysis to the project and resolves compatibility issues to ensure proper support for PHP 7.4+. The changes improve code quality, type safety, and maintain backward compatibility with older PHP versions.

Changes

🔧 Static Analysis Setup

• Added PHPStan as a development dependency (version ^2.1)
• Configured PHPStan with level 3 analysis covering src and tests directories. Level 3 is very low but only this level can be achieved with minimal effort.
• Added ext-json requirement to ensure JSON extension is available

📦 Dependency Management

• Downgraded dependencies to support PHP 7.4:
  • PHPUnit: ^10 → ^9
  • psr/log: ^2.0 → ^1.1
• Updated platform requirements to explicitly support PHP 7.4 and 8.0+

PHPStan Configuration

The current PHPStan configuration uses level 3, which is intentionally conservative to establish a baseline for static analysis. This low level was chosen because:

• The current codebase uses array-based configuration patterns that don't provide strong type hints
• Many constructor parameters are passed as array $options without specific type definitions
• Higher PHPStan levels would require significant architectural changes (Discussion: Library architecture and dependency injection #6)

[fahad19]

thanks @Greg0!

will merge and release if I have your approval.

[Greg0]

Yes, it can be merged