PHP - Switching between websites? - dns

Question

A friend of mine set up a website with a website builder. I offered to help and ended up redesigning the site. What I wrote is obviously incompatible with the site builder's templates he used, but he would rather stick with the site builder which allows him to make the changes he needs to make.
What I'm currently thinking is to have both sites set up simultaneously and let him choose which one is visited by the user. That way we can use the new site and then when he makes a change, switch back to the old site until I have a chance to change the new version. This would be confusing for any regular users of the site who happened to visit it in the time that the old site was changed, but the site is very small with very few visitors, and any changes made would be easy to duplicate.
I don't, however, know how to do this. My first thought was to have two public_html directories with a script that would rename them as needed, but I don't know how I would go about changing the name of a folder that a script is running from. I also don't think I would be able to move the old site over to a host I control; the site builder doesn't give the ftp details and it takes care of all the dirty work for its users. At most, I think I could transfer the domain name to a different registrar, set up the nameservers with the host of the new site, and go from there, but then the old site wouldn't be editable from the site builder...
My second thought would be to do something like this in php:
if(...){
echo file_get_contents(oldsite . $_SERVER['REQUEST_URI']);
}
else{
...
}
If anyone's made it this far, here are the key points of what I want to achieve:
Old site is able to be changed with the site builder (vistaprint)
Script to choose which website the user visits
Urls never change
Assume the only control we have over the old site is a simple WYSIWYG editor from the site builder and the ability to change the domain as well as release the domain (for $20, the crooks). Nothing to do with nameservers, ftp, file managers, etc.
Current plan of action: Transfer the domain (lets assume it's http://example.com) to a register and webhost I control. Set up an add-on domain (http://old.example.com) and register the web builder with the addon domain. I'll then have each page set up the way I showed in the second plan:
if(...){
echo file_get_contents('http://old.example.com' . $_SERVER['REQUEST_URI']);
}
else{
...
}
I'm a bit reluctant to do this for a number of reasons, so I was hoping someone could enlighten me on a better method.

Answer 1

You're overthinking this. Just redirect (HTTP status 3xx) to your old site when necessary.