I was practicing as a web designer during the beginning of the "web 2.0" era and picked up a few tidbits of information along the way. I won't say that I'm professional but I will say I know more than the average bear.
First and foremost I think you might know this already (or maybe not) but HTML5 and CSS3 is all the rage and is appearing all over the web one site at a time. It seems video websites are the largest of the early adopters of the new craze due to mobile devices and the limitations of processing power and accessibility with the flash platform. (namely, Apple and their iPads, pods and phones)
Next, I should also tell you that the web isn't just HTML, CSS, JavaScript and PHP. No, it's much more than that. We also have XML, Databases (SQL-based mostly as far as I'm aware) and various "techniques" using a combination of the above scripting and programming languages. You also have to worry about search engine optimization, the document object model, common browser rending bugs (especially with internet explorer) and that there are standards in place used by almost all professional web designers which were developed (and listed) by the
World Wide Web Consortium a.k.a. the W3C.
You also must note the fact that the web is currently going through a paradigm shift. We are shifting from "the social web" (Web 2.0) to "the semantic web" (Web 3.0) which is where everything has a place, context and is properly organized...in other words, the web is turning into search engine crack.
Finally, you might want to know that the web isn't programmed only in PHP, in fact some of the biggest websites avoid it all together as it doesn't scale efficiently. Ruby on Rails is used by Twitter and LinkdIn. Python is used by Google and YouTube. However, PHP is in fact used by big websites. The biggest? Facebook.
Oh, wait, that wasn't the final note. Choose your CMS early on. Over the years I've tried dozens (if not over a hundred) CMS's and I've primarily focused on three of them. Wordpress, Drupal and Joomla.
Wordpress by far is the easiest of them all, but is geared towards blogging websites. Drupal by far is the hardest, but the most powerful once you learn how to use it right (MTV and The official U.S. white house website run it) and Joomla, well it's the perfect combo of ease of use and extensibility.
Personally, I used Drupal. Always have, but soon will be looking at something new as I'm going to switch to python soon.
Ouch, and wait there is more. Frameworks...frameworks.
CSS and JavaScript are known for having their frameworks. For CSS I recommend the yahoo YUI library and Blueprint. For JavaScript I recommend (one or the other, never both) jQuery or mootools. and last but not least, always remember that IE6 and 7 are still widely used, and still horrible to work with. So expect frequent headaches.