What are the different between $this and self in PHP?

While I was reading [static keyword](http://www.php.net/manual/en/language.oop5.static.php) on [php.net](http://www.php.net), I got confused with the following sentence.

> 
> $this is not available inside the method declared as static.
> 
> 
> 

Why $this is not available while doing so?

Any help will be appreciate!

Thanks!