PHP - How to get info from a newly created user

Im totally new to php but have been programming for around 5 years in C#.

Im struggling to get my head around some of the basics in this.

So i have an application that will take info from a user to create a new entry in my db for their new account. Once the account is created i then need to know their unique id.

Should all of this be done in one php file? Or should i create another php file to grab the id?

If i take the second approach how do i know what the email was they used in the first php file? In C# i get you can create references, and reference other variables but how do you do this in PHP? In my understanding if i use the include in any file it will execute all the code in that file?

Thanks

John