I was working on fixing a performance issue on a page that requires a login to work. To work through the issue I wanted to send a bunch of requests to the page using ab but need to have the user logged in. In order to do this you can specify a cookie value using the -C parameter:

ab -C PHPSESSID=co1d400krhndtumi5orsfr70c4 -n 10 -c 10 http://hostname/controller/action