1) As far as I can see, nothing. Apache is a server designed to handle HTTP requests and return a page, using the driver it deems necessary. One of these is PHP. MySQL is software which PHP has, again, drivers to access. They're used in web development, but so is a web browser (developing without using the browser regularly is like painting without looking at it regularly).
2) Open Source is generally a competitor to commercialism. Open Source applications are growing as fast as commercially built ones, thanks to the people who (a) gain benefit from product growth, (b) want their name on (potentially) widely used software, (b) just want to do some honest good for others or (d) have too much spare time on their hands. So essentially the main benefit it is often allows cheaper/free software of similar quality, and it also saves the energy used in producing, transporting and disposing of the tissue paper that many of the regular contributors under category (d) would pretty likely be using up with their otherwise spare time.
3) It makes data storage, managing and retrieval easier and faster. What I want to know is this: What is the purpose of a duvet? Why do humans have such a comfort requirement when dogs are happy with what they also wear throughout the daytime.
4) 2^16 - 1.
5) False. It should always have a non-null value. I suppose one record may have a primary key equal to null but, being unique, none of the others should.
6) Lots of variations. You're probably thinking of INNER JOIN, but LEFT JOIN, UNION and nested queries TECHNICALLY join 2 tables. Just with different results.
7) Typos, bad colour schemes and using XHTML without actually serving it as XML.
8) When you have had too much coffee and not enough sleep. Or when you're using an outdated PHP version and are trying to use namespaces.
9) Sleep more. Drink less coffee. Pay attention to what you're writing. Remove oven gloves before typing, after cooking.
10) True, kind of. The image function is handled by GD, so is technically C, but PHP calls the library.
Battery is dying on the laptop, so I will continue tomorrow if I remember. I'm annoyed there were no architecture or library development questions. Got my hopes up a bit...