Do apple iphone cases have a warranty?

Short answer: No.

A bit longer answer: If you have an AppleCare+ plan with your iPhone (not the case), accidental damage is covered but only provides reduced repair costs. The basic one-year hardware warranty only covers manufacturing defects.

AFAIK, using an Apple case does not have any added advantages over the AppleCare+ plan as far as reducing repair costs.

On the other hand, some third-party suppliers of cases do offer some form of repair cost reduction benefits.

normal warranty only covered production defects discovered after purchase and never loss, theft and user caused damage, it's not as such an apple thing but the general nature of the warranty system

Legal - Hardware Warranties - Apple

and

Warranty - Wikipedia