Joshua Thijssen has a new post today with a "neat trick" that the Symfony Security component allows - switching (impersonating) another user programatically.
This allows you to login as another user, without supplying their password. Suppose a client of your application has a problem at a certain page which you want to investigate. Sometimes this is not possible under your own account, as you don't have the same data as the user, so the issue might not even occur in your account. Instead of asking the password from the user itself, which is cumbersome, and not a very safe thing to begin with, you can use the switch-user feature.
He talks about how to enable it, how to use it to switch to another user and, most important, how to restrict its use. He points out that there's no way to define who a user can switch to built-in, so he's come up with a custom "switch listener" to help add in this protection. His "SwitchUserListener" class replicates some of the code in the original handling (well, the whole class) and updates the "attemptSwitchUser" method to check the user they're trying to switch to and see if they have the right role. Finally he shows how to add it to the services configuration and how it overrides the default listener.