Can a PhD doctor declare someone dead or does it have to be a medical Dr.?

In the UK surprisingly there is no legal requirement to verify the fact of death. In practice this is done by any doctor, but can be an appropriately trained nurse or ambulance person.

The Medical Certificate of Cause of Death, which we usually call a Death Certificate, can only be issued

by a doctor or by the Coroner. There is even provision to say that the deceased was not seen after death by a medical practitioner, and I have linked a specimen MCCD.

A non-medically qualified PhD is no more qualified than any other member of the general public.

Doctor

Depends on the setting. Out in public medics can declare someone dead as well. If lividity has set in or the patient has injuries incompatible with life, or something else that makes it an obvious death. If someone witnessed a cardiac arrest most likely the medics can get there quick enough to attempt life saving measures.

It has to be a medical doctor.

Although PhD holders can use the title "Dr", they aren't actually medical doctors or healthcare staff, so most of them have none of the rights or responsibilities that healthcare workers have. Most PhD's have nothing to do with medicine or health.

Candy a medical Doctor by Law.

Medical Doctor -- but a cursory internet check will show many mistakes, though the percentage is likely low, some banging coffin or struggling in zipped body bag, or about to go into crematory.

It has to be a Medical doctor.

Must be a medical doctor.

I would assume only a Doctor with medical knowledge would be able to say a person is dead

A PhD cannot; they have no medical training what so ever. The state statues dictates who can pronounce death. A MD/DO always can, in some states a PA can, a paramedic can in certain situations, etc.

a medical doctor or the cornorer