Forwarding subdomain to main domain

Forwarding subdomain to main domain

So I'm not sure my title makes sense, but here is what I'm trying to
accomplish: My programmer created my WordPress based site at
xx.mydomain.com Everything on this subdomain works perfectly.
A) is there anyway to just have mydomain.com automatically forward to
xx.mydomain.com? Should this be done with a simple php script in the index
file on root or through some code in .htaccess?
B) is it ok to leave it at xx.mydomain.com? Will it create problems with
search engines? At some point I will have yy.mydomain.com. At that point I
will create an index file where users can select xx or yy.