A demo 1st site in php will be provided, which is pretty simple.
Now, copy out only a few pages from the 1st website onto a 2nd site. Adding only a few pages will do, including login & registration page, and edit profile and Show profile page.
Next, on 2nd site, using Json, users there to be able to register & login to 1st site, & show and edit their profile information on 1st site, "all without leaving 2nd site, ie by being on just 2nd site itself", and without going to 1st site. Json to query MySqlDB on 1st site using db Username and db Password, and create session to hold or update all data pulled out from 1st site, through 2nd site. Use Json to send or interchange data between the 2 sites, including stringify and parse and session.
You will work on my localHost directly. Video recording software tracking what steps are undertaken and recorded for future reference (if need be). TeamViewer, or Anydesk, etc password will be provided. You can just leave the video running in the background, un-interrupted.