docs: add Elephant.IO as PHP client library (#4779)

Author: tohenk

Readme.md

@@ -22,6 +22,7 @@ Some implementations in other languages are also available:
 - [Python](https://github.com/miguelgrinberg/python-socketio)
 - [.NET](https://github.com/doghappy/socket.io-client-csharp)
 - [Rust](https://github.com/1c3t3a/rust-socketio)
+- [PHP](https://github.com/ElephantIO/elephant.io)
 
 Its main features are: