PHP from hosting in to VPN server

I have a php enable form which is currently running in linux shared hosting in which I have many logins for salesman, accounts, units and dispatcher. Its a order management software. Now I want it to make more secure like I want only one can open whom I allow to access. otherwise no one can open it with the url. I can't make it window based as my sales person is active on mobile and from other states in India.