Redirect HTTP requests based on subdomain address without changing accessed URL?

Let's say I have a domain, www.mydomain.com.
And I ordered a new domain, abc.newdomain.com.

Both domains are hosted in the same ISP, so currently requests to either of those addresses result in the same page being shown.

I want to redirect all requests to abc.newdomain.com to folder /wp so that when users access abc.newdomain.com they would see whatever is inside folder /wp without seeing the URL change.

Questions:

1. How can I achieve this using .htaccess?
2. How can I prevent users from accessing directly /wp directory (meaning that www.mydomain.com/wp would be blocked)?

From serverfault tputkonen

• mod_rewrite can do both of those.

  For the first, check the host with RewriteCond and write a rule to rewrite it without using the R flag.

  For the second, check the host for www.mydomain.com and write a rule that rewrites anything back to itself with a R=404 flag.

  From Ignacio Vazquez-Abrams