Browse index after login in with php

i'm working on an E-commerce website and i'm willing to do the back end with PHP, so there is an index page (default) which opens after you browse the website's link which contains the offers, products etc.. so there is a navigation bar with sign up and log in buttons, what i need to do is after the log in i want the same index page opened with some differences (log in button replaced with account button with a profile picture for example and the sign up replaced with log out button) without going to another directory.. how can i do that exactly please ?