How to cap server load?

[KT Walrus]

I really want to ensure that my web servers are not overloaded.

Can I do this with nginx?

That is, is there a variable I could test to decide whether nginx should send the request to the local PHP backend or to forward the request to other nginx servers in the server farm, based on the load of the PHP backend?  Maybe a variable that contains how many concurrent requests to nginx are waiting for a response?