How can I delete all iPhone photos without deleting them from iCloud?

iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See the following Apple article for more information: Set up and use iCloud Photos - Apple Support

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.

PaulinPR wrote: …How can I delete all iPhone photos without deleting them from iCloud?

You'd have to disconnect the phone from iCloud, and then delete the pictures. But if you ever reconnect the phone to iCloud, you'll get all the pictures back. As FoxFifth and razmee209 said, iCloud Photos is a synchronization service. Using iCloud ensures that the Library of pictures on all of the devices connected to your iCloud account will be exactly the same. If you add a picture to one device, it will appear on the others. If you delete a picture on your iPhone, it will be deleted at iCloud.com and on any other devices you have. Of course, by deleted we mean that it can still be recovered within 30 days unless you delete it a second time in the Recently Deleted view. 

However, you can turn on "Optimize Storage" to keep only smaller images on the phone, potentially saving 89%-90% of the space.

PaulinPR wrote:

I have 2000 photos that I want to keep in the cloud. I have 2000 photos that I want to delete from my phone to free up phone storage that is full. How can I do this?

You don't since you're syncing your photos read here Set up and use iCloud Photos - Apple Support