Internal App for a website

We want to create an internal App for our website. It will be a simple, basic app shown below.

App Side*
==> Login
==> Leeds List (Apply Booked)
==> Customer List
==> Booked List


*API Side*
==> Login API
==> Leed List API
==> Apply Booked (Date & Time)
==> Customer List
==> Booked List

It is just for our own use nothing commercial.

The site's admin is in PHP

If you have done something like this please show examples in quotes.