i need to back up my iphone but it is cracked

Your old phone does not back up to another phone, it backs up to iCloud or to a computer.

If the old phone is damaged and cannot back up, do you have a recent backup of the old phone on iCloud or a computer that you can use to move data to the new phone? If not, there is nothing more than you can do.

Then there is nothing you can do about it. You can’t backup now.