First thing first. You should learn PHP and there is nothing in HTML which you can't learn from doing PHP.
Its the programming which you need to learn and how to put images / content in pages is HTML which is very basic stuff if you know somewhat of PHP.
Now after knowing PHP it would have advance where it deals with database and as soon as you have it you would start building your websites and apparently see how HTML and CSS things work.
Definitely starting with HTML/CSS is theoretically correct but I would like to see more practical approach.