After Login, redirect to another page

[jtaco263]

hellow i have my domain like mydomain.com and roundcube runs mydomain.com/mail/ im using clevercube plugin so people can log from mydomain.com however i need php code so if the user is logged in cant access parent domain (mydomain.com)
in a few words if the user if logged in and reaches the url mydomain.com this redirects again to mydomain.com/mail/