This is an easier method of setting variables for the session cookie. It available in versions above v0.6.3 (1.0 betas and higher). Examples assume your front application name is called "frontend".

config/frontend/factories.yml:

storage:
    class: sfSessionStorage
    param:
      session_name:            MYAPP_SESSION
      session_cookie_lifetime: 77760000   # 90*24*3600
      session_cookie_path:     /
#      session_cookie_domain:   localhost
#      session_cookie_secure:   true

Comment/Uncomment what you need. That is all, there is no need to create a custom class. You may need to clear symfony's cache.