It is worth looking at the PhD by published work route.
This means that you could start researching and trying as a goal to get something original published without needing to apply, be registered or pay any fees.
At some stage registration would be required and probably a thesis-like document prepared explaining the relevance of the publications and so on, paying a fee and an examination, but you could get started now on your own initiative and if you get anywhere with it get retrospective academic credit for what you have done before the formal registration.
Regulations vary greatly from university to univeristy, some only for their own academic staff, some for their own academic staff and their own graduates, the graduates sometimes only becoming eligible to apply after at least a number of years have passed since graduation, yet some allow any graduate, not necessarily from that particularly university, to apply.
As lots of source material is now freely available on the web for some subjects this could be a good option for home study.
It depends which subject interests you, such as whether you would need access to an equipped laboratory or not.
Please note that the oral examination can take place anywhere agreed, so potentially, if someone needed it, in an office in a care home where the candidate is living at the time, with the examiners visiting.
It is known by various similar names at various places, but searching for PhD by published work on Google gives lots of links.