在PHP登录脚本中使用会话和会话变量

I have just finished creating an entire login and register systsem in PHP, but my problem is I haven't used any sessions yet. I'm kind of a newbie in PHP and I've never used sessions before. What I want to do is, after the user registers and fills out the login form, they will still stay on the same page. So, there will be one part of the which will be if the session is logged_in and the other part will be else (the user is not logged in so display the login form). Can anyone tell me how to get started?I have just finished creating an entire login a