Deploying another application as subdomain

Let’s say that I have an application up and running at http://eii.com

I and developing another application that is complimentary to eii.com
and
have decided to serve that application using http://services.eii.com

Now this is a totally different application but is useful to the people
visiting eii.com site. This new application has it’s own database and
own
source code.

My problem is login part.

If a user logins to eiil.com and then visits services.eii.com how do I
know
who the user is and if the user is logged in or not? Should I use
cookie
for this purpose then the someone can change the user_id in the cookie?

Thanks

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