WordPress Authentication

I have created a project in PHP and i am trying to integrate wordpress login in it, as this project is related or dependent on wordpress.

Lets say i have 5 pages in this project. On every page i want to check if the user is logged in, if not that redirect him to wordpress login page.

So i need to know the wordpress authentication function that i can put on all the pages and check during page load.

Thanks for your help.