I know very basics for all these languages html, php and css. I have pretty much covered all the HTML I need for my sites already and use Dreamweaver for general updates. I want to develop my php skills further, would you say that it is essential to know html fully before I go and develop these skills? Is it important to my understanding of the language? Or will it not benefit me at all to learn better HTML before PHP? Thanks in advance.
Learn HTML tags as you need them. Learn PHP functions as you need them. You will never know everything about either one so look at what interests you more.
View the source of this page and be sure you understand what each tag is for. Don't worry about the code between the script tags or the files certain tags are pointing to, just be sure you know the purpose of each tag in the source of this page. Look up all the ones you don't know or aren't sure of. That should be enough to get you up to speed on HTML. Next, pick up a PHP book with a project in mind. Read the first 5 chapters or so, then start on your project. Maybe its a clone of craigslist, maybe you want to make a website that displays random quotes, whatever it is, have a specific project you want to complete. Then make your project happen using the book and php.net as guides.
PHP is what will take you the longest and really has a lot of stuff you need to understand. It's progression is sort of linear, in that there are prerequisites to understanding parts of PHP. HTML and CSS you can kind of pick up as you go along. I would start with PHP and pickup the HTML and CSS when they crop up.