If I buy an app on my iphone will I have to buy it again if I buy a new Iphone?

Next month I'm getting an Iphone5s currently on my 4s I have a few purchased apps.

Will I have to rebuy it when I get the 5s?

If you login on the same itunes or appstore account no

No. They are assigned to your Apple ID which will work on any future iOS devices you plan to buy.

Not if you know what you're doing.

You won't have to buy it again. It checks which apple ID bought the what app. If you buy it once you don't need to buy again. You are just downloading an app that has already been purchase into another iPhone.

No, just log into your Apple account again on the other device
Your apps, in-app purchases, songs… Etc. Get saved on your account