[RFC] Sentinel PHP version

## Issue Description

Type: *feature request*

### Describe what feature you want

Implement PHP version of Sentinel, including (but not limited to):

- Basic statistic structure (sliding window that can achieve high concurrency and accuracy)
- Rate limiting
- Circuit breaking based on error percentage or average response time
- Logging (both common log and metric log)
- Integrate with Sentinel dashboard (transport module)

Discussions are welcomed!

No files, tests, or entry points are named. Start by reviewing the existing Java Sentinel implementation and the requested areas—statistics, rate limiting, circuit breaking, logging, and dashboard transport—then define the PHP integration boundaries and acceptance criteria for each component.

Written by the indexing model from the issue text.