How do I go about making a subdomain appear to be a subfolder?

I’m trying to find out how to make a subdomain appear as a subfolder ie:

if I have -

forum.example.com

but I want to browse to -

example.com/forum

to find it rather than have it available at forum.example.com

also - how would I work this with SSL?

should I add forum.example.com to the certificate? or would it be covered under example.com?

Thanks for any pointers!

Johnny