Fix: PHP SDK Deprecation Warnings and API Implementation

[devin-ai-integration]

Fix PHP SDK Deprecation Warnings and API Implementation

This PR addresses the deprecation warnings in the PHP SDK and improves the API implementation to match the JS SDK structure.

Changes

• Fixed deprecation warnings about implicit nullable parameters by using explicit nullable types (?callable instead of callable = null)
• Improved error handling for API requests with better error messages
• Enhanced parameter validation for all methods
• Updated API request format to match the JS SDK implementation
• Improved progress callback handling
• Added validation for reference parameter format

Testing

These changes have been tested with the PHP SDK demo and ensure that:

• No deprecation warnings are shown when using the SDK
• API requests are properly formatted to match the JS SDK implementation
• Error handling is improved with more descriptive error messages

Link to Devin run: https://app.devin.ai/sessions/7e5ae8c97d464b8480760363c72c0712
Requested by: Max Prilutskiy (max@lingo.dev)

[devin-ai-integration]

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