So I'm building an application which exports contacts from various CRMs and unifies that data to run marketing reports on their contacts.

I was easily able to get HubSpot and InfusionSoft working with their Oauth2 apis, and I was hoping that salesforce would go just as easily.

My desired flow is to redirect the user from their private domain (e.g. `user1.appDomain.com`) to the salesforce site to log them in.
Then I want to redirect them back to their private domain (`user1.appDomain.com/oauthReturn`) with their access token so I can start importing their data.

I registered an external application in my developer salesforce account, and have received my oauth tokens, but I need to know a few things (that googling hasn't solved yet).

So I need to know a few things first.

 - Will salesforce allow me to pass in the redirect url from the user's private domain?
 - Can I even do this with Oauth2 and php? The documentation is unnecessarily confusing, and most of the libraries are still using api tokens and all of the relevant documentation I can find is 2-3 years out of date.

If anyone knows where I should be looking to find these answers that would be extraordinarily helpful.

Thanks so much for your help!