HTML is like said chris said, a markup language, check the M in HTML = markup,
CSS defines the style of your website.
PHP is a scripting language.
Dont learn html, learn XHTML from the start! it's almost the same, though some more specific rules!
xHTML is just as inconsistent (if not, worse) as HTML. The newest standards-compliant web markup language is the one you should learn (if you learn any at all). Right now that is HTML5. Remember that mark-up languages were designed for hand-coded, error-prone word-documents- so they have a tendency to "just work" even if the documents are full of errors. For that reason, mark-up languages have a tendency to appear differently when loaded by different interpreters (today, browsers are the interpreters). Mark-up was designed for static, motion-less documents. Somebody added the "hyperlink" and now we're stuck using a very unfit technology for web-sites.
CSS is a cascading stylesheet language invented for styling printed documents. It is still used and developed today for dynamic web-sites- despite the fact it's a horrible tool for that job. The latest version of CSS is CSS3.
You can't go wrong learning JavaScript- the most popular programming/scripting language. However, JavaScript has it's quirks, too. Originally developed in 10 days by
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at Netscape, JavaScript has many bad parts. Unlike HTML and CSS, though, JavaScript is the only language we use in web development that was originally designed to be used in a web-browser. The newest revision of EcmaScript (EcmaScript 5)- is what you should learn if you decide to learn JavaScript. [EcmaScript is the subset of JavaScript that is not dependent on any web browser or the DOM]. If you plan on using JavaScript for web-browwsers, you may still want to make your JavaScript code compliant with EcmaScript 3 in order to support Internet Explorer 9/10 and below (not sure about 10 yet) and other legacy browsers.
Yeah, we got it bad here at the web, I suggest going to Python which is very clean, concise and easy to learn.. Or if you want to dig deeper into computers, start with C. It won't hurt much to learn both C and Python at the same time and see which one you really want to start with.. No sense punishing yourself with C++ so early on in your journy.