done it, been there, all langauges;
HTML: this is the code that makes something appear on the screen, like a form, or tables
CSS: Can be used to secure html a littlebit more and css is like a rope, you can hang things in it, and place stuff on the exact right place you want.
PHP: serversided langauge, that can process serveral actions on page upload and does not get send over to the client, which allows you to alter your webpages through the use of variables. ( as you might know, a website host, uploads the page code to the client's browser, where it gets processed)
php means Hypertext Preprocessor, in other words, it processes php code before it sends it over, allowing you to alter the html code, so you can manipulate certain text pieces or display a table with information.
php can be used to create forums, blogs, login-register systems, news systems, site encryptions, actually everything you can imagine that is within php's power.
i also recommend you to learn php, especially the forum building part. once you learned how to build a forum, i recommend heading up to using sockets. after that, you should be able to build anything you like.
but before you learn php, you should learn CSS, so you can place certain pieces on your site which you can't now because of the lack of power within html.
anyhow , my recommended learning order:
HTML:
CSS:
PHP:
JAVASCRIPT:
javascript can be very usefull aswell, but don't forget if you also want to learn this, that this CAN be disabled by clients.