Haha, your little Emily sister is here. Mwahahaha.
You can embed php into html code with <?php and ?> tags. In between, you write in an entirely different language than html.
www.php.net is a good resource, though it might be a little intimidating if you're new to programming. Try PHP for the World Wide Web by Larry Ullman. (It's a book, in case you didn't pick that up.
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Many if not most websites you see with logins, user accounts, etc. are done with php. Once you get started there's a lot you can do with it.
(I could get paid to advertise this stuff, couldn't I?
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php? 
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