Thanks for this! It wasn't working for me initially, and asked around. looks like you missed a "]" in the first line. So it should be:
if(isset($_GET['redirect'])){
I was able to get it to work at the root domain. A.com --> C.com
After getting this all sorted out though, I realize that I really wanted a pop up screen that could be closed, and that it would require me to spend even more time on it, and trying to add javascript as well. I decided to just scrap the idea.
Another problem I had was that the ?redirect=true at the end of the URL, messed up the direct linking to subpages and posts as well. So instead of :
C.com/blogpost-20 I would get C.com?redirect=true/blogpost-20.
Since there is no such page, it just returned the contents from the home page.
I appreciate your help though. It just seems like more effort that I want to put in, and probably more than the client wants to pay for.