If I want to build an admin area, to change prices the customer for his RentACar website what to do so this "web page:change prices" accessed via a password only and not directly by anyway ? I already have a secure booking system for the website(https://) ?

Probably you want to have apache basic auth

I just want the page access via id/password (login.php) page eg admin.php to be not directly accessed ? what to program on admin.php ?

In the priceslist.php I mast retrieve prices from db (and show), samely with processquoteprocessor.php (and process quote's inserted dates) ? Well I think problem as I see is modify prices after login, how to go to modifyprices.php ONLY after login and NOT directly by typing the whole URL ?