Apologies for the necro, and correct me if I'm wrong, but what it sounds like you are attempting to do is called Single Sign-On (SSO). Users log into your main app and, upon accessing the forums, maintain their login.
I had asked about this in the thread below, but at this point, nothing exists to allow either SSO or LDAP functionality, the exception being SSO capability across multiple installations of xenForo, though I believe the intent with these plugins is to allow for some rudimentary load balancing.
https://xenforo.com/community/threads/xenforo-active-directory-ldap.111814/
As this is a fairly specific SSO application, you may want to look at hiring a developer to provide you with this functionality.
If this is a business solution and you are able to get both symfony2 and xenForo to support LDAP authentication, then you can allow pass-through authentication. In this case, it's not using cached credentials stored in an encrypted cookie to log you in, as SSO applications are structured, but the web server is querying your Active Directory object and automatically logging you in.
Unfortunately, the above solution is not feasible within a paid hosting environment, but it's food for thought.
If you have any PHP experience, you may want to look at the xenForo SSO plugins and see if you can modify their code (assuming they make it available) to connect to your symfony2 server. Other than that, I am out of suggestions.
Hope this helps,
Fox
EDIT:
...nothing exists for either SSO or LDAP functionality...
As shown in this thread:
https://xenforo.com/community/threads/oauth-2-openid.114502/
You can use oAuth2.0 authentication, which is supported by Active Directory Federation Services.
https://msdn.microsoft.com/en-us/library/dn633593.aspx
Additional development would be required, as pointed out in the thread above, as the current oAuth2 functionality only supports Google, Facebook, and Twitter. However, if symfony2 allows for oAuth2 authentication from services other than those providers or allows for said additional development, then this may be able to provide you with your solution... again, assuming the webserver(s) hosting symfony2 and xenForo are on the same network as a Microsoft Active Directory server. But hey, at least it's possible.
