When I first looked up this question starting out I saw so many good answers but to sum it up:
1) Think about how much you would like to earn per year, divide it by 52, then 5, then 8 and there's your hourly rate
2) Next thing to consider is, is that hourly rate reasonable? Check out job adverts for graphic designers or jobs in your field and see if these match up to your annual figure.
3) Sometimes your hourly rate doesn't always stay the same as you may have to fight for a job, therefore it would be a good idea to look at what is the lowest you would go. Make sure it's a figure that will still cover costs and give you profit, no point in getting nothing out of a job but still a point to get the job in the first place for possibly more work from them in the future.
4) Be true to yourself. If you can't do that and you are sure that you are brilliant, you are probably wrong, in which case put your work up for critique and ask not just whats wrong with them but of what quality the work is, then you can decide at what level of expertise you are.
By 'HTML' do you mean full front-end coding, CSS, standards compliant coding, etc?
Knowing just some PHP won't have much commercial value, but it's a good foundation for you to build on and gain knowledge. These days there's a lot of non-taxing PHP-related work in CMS implementation, customisation and add-on development.
Designing and building web sites involves multiple-disciplines requiring two opposing mindsets. Are you more of a concept/graphic designer or a dev/code-head? On the design side, code-aware designers are more valuable.
Let's suggest some rough ballpark hourly rates for experienced people: