.htaccess ?

Seriously, your question is too vague for a qualified answer : Let me google that for you

Using a framework such as cakephp or Laravel could help you significantly as they have their own authentication libraries built in and it's really not that hard to learn.

In all seriousness though, if you don't know how to make your own authentication system in PHP, I wouldn't suggest trying to make one and putting it into practice. PHP is a language where you can easily make one little mistake turn into a huge disaster for your website. Having said that, it's a great language to learn if you want something easy to use and on the go. Take a look at the frameworks I mentioned above .