saving source code of another site to a variable

I'm trying to save the source code of another site to a variable, but I have on idea on how to do it! Can I get some help?

That cannot be done.

For security reasons JavaScript cannot access anything beyond the domain of the current page. This is known as the "same origin policy" and prevents a document or script loaded from one origin from getting or setting properties of a document from a different origin.

I have seen other sites do similar things, what code did they use?

use java or php, that is, server side languages.

Thanks!