Does state law trump city law?

In this particular case, state law is absolutely clear. The law specifically states that special parking for persons with disabilities "shall measure five feet" to the top edge of the sign. The word "shall" is legalese for "must be without exception (unless exception noted)". City ordinance cannot change the state law in this case since there is no exception noted stating that the law can be altered.

State law doesn't automatically trump city code. Also you don't say that the state law says that signs must be 5' and ONLY 5' and no higher placements are allowed. If you bring pictures of the branches blocking the sign you might have a little tiny bit of an argument, but I'm assuming that the ground was also painted with blue lines too...

Generally, state law trumps city law. However, it's possible for a city to have a law that is MORE restrictive than the state law, and that would be enforceable.

For example, many cities have laws that forbid pet breeding, even if no such state law exists. These laws are enforceable. As another example, some cities have a minimum wage that is higher than the state's minimum wage.

In your case, the state law seems to be more restrictive, and therefore would be the one to be enforced.

Yes, and federal law trumps state law.

But, I think you're reading the law wrong. When it says that parking signs must be 5' off the ground, that's probably a minimum, not the exact height a sign MUST be.

Try looking around more dilligently for signs. I'm sure if it were a sign saying "If you park here you win a million dollars", you would have magically been able to see THAT sign.