Authorization & multiple 'docs'

hi i have Apache with PHP4 and PostgreSQL installed. i'm writing a simple internet app. and i have some basic Qs. i have users listed in SQL base with names and passwds. i've made an PHP page, which checks if correct names and passwds are writen in a form. if the name and passwd are OK, it calls up the second php page. problem is, that any user could access that second page without authorization, simply with directly typing in it's name! How can i avoid that? how can i restrict access to that other page from pages different that previous (if it can be done?)? grateful for any help thnx Robert