I see that many iOS games nowadays don't hardcode their store, and items, description, price etc. are loaded from an external server.
What would be the easiest way to implement this? I am a game programmer with very little experience in server side programming. (Have done hobby PHP scripts a long time ago)
Please let me know what libraries can ease the effort on server side / client side. I would like something that is easy to manage. How do they announce offers like 50% off on certain items etc. whenever they want? Doesn't every in-app purchase manipulation need to get approval from Apple?
Also I would want to maximize security, and prevent the game store getting hacked as much as possible.
You are correct that this has to go through Apple's IAP. You'll want to read about this at https://developer.apple.com/appstore/in-app-purchase/index.html. You setup the IAP items in iTunesConnect (iTC). Your in-app store lists only items that you've setup in iTC, though you can choose not to list every item in ITC. To make your store dynamic, the easiest way is probably to use a UIWebView and then have your store be a series of webpages. This lets you update it on your server easily.
You might check out http://stackmob.com which makes it relatively easy to do just the store part of your in-game store without having all the server admin aspect of it (and associated security). Also, http://urbanairship.com provides hosting for IAP items.