Hi,
Obviously I'm new here, and fairly new to PHP - so far this book has been very helpful and usually easy to follow. I got the 3rd edition from the library and got through Chapter 10 before ordering the 4th edition when it came out. So now I'm just skimming back through the early chapters to see what's different.
The calculator script is quite different, and I was wondering about the method of checking if the form has been submitted. The earlier version used a hidden form input and checked if the value was set, the new one checks if $_SERVER['REQUEST_METHOD'] is POST. What's the difference? Is that better or more secure? Does the hidden form input being visible in the HTML source have anything to do with it?
BTW, I like your way of showing the tags in blocks. On a couple art websites I'm a member of it's often difficult to tell how tags are going to show up until the piece is posted - and that's important if you're trying to sell something!