maicol07 I did but did you test your solution just when the forum is at mysite.com/forum/ or also when it's at forum.mysite.com ?
Because I don't think you can unset a cookie which was set from forum.mysite.com (set by the flarum application) from mysite.com (different php application). I think that would be a security issue?
I think in this scenario you would need a redirect from mysite.com -> Logout -> to forum.mysite.com to unset the session cookie there, then redirect back to mysite.com.
Or modify your flarum plugin to set the flarum_session cookie to .mysite.com instead of forum.mysite.com (but I'm not sure if Flarum supports that from within an extension?). Then it would work I imagine.