173 is effectively a collection of ideas for puzzle levels that I came up with while working on other online puzzles. They were stored in a folder on my computer and slowly accumulated over time. Eventually, I had enough to publish the whole thing online.
Nothing much; the website was named spontaneously.
Flexibility. To cut a long story short, you can do interesting things with a .php that you can't do with a .htm.
Yes. Sometimes. Sorry.
Level 3 was created in an effort to demonstrate a less common way of progressing from one point to another in an online puzzle. Some may consider it a sudden difficulty spike, while others may solve it in under a minute. What I can say is that if you are completely lost on what to do, consider reading the answer to the third question on this page again.