What is the best PHP MVC framework for scalability?
There are a lot of MVC frameworks for PHP. **Which one is the best in terms of scalability?** Is there any framework that supports splitting applications on many front end and database servers? Sharding? Other enterprise features?   
Ideally the framework should support sites with thousands of concurrent users per server.

I would greatly appreciate if you could specify if your opinion comes from experience and what level of scale was supported by the system.
What is the best PHP MVC framework for scalability?
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There are a lot of MVC frameworks for PHP. **Which one is the best in terms of scalability?** Is there any framework that supports splitting applications on many front end and database servers? Sharding? Other enterprise features?   
Ideally the framework should support sites with thousands of concurrent users per server.

I would greatly appreciate if you could specify if your opinion comes from experience and what level of scale was supported by the system.