Deferring PhD offer

As others have said, this depends on country and discipline, and to some extent on school/department itself. So we can't give you a trustworthy answer.

In places I have known (all North America, various disciplines), it was no problem to defer admission itself for up to one year, none or very few questions asked, in programs where you had a year or two of coursework to do before settling down to 100% research. It was also not a problem to defer cohort-based, non-competitive departmental funding. And instances I knew about, also not a problem for the applicant to negotiate (separately) deferrals of external funding they had obtained.

However, two years would have been a special case, and subject to discussion. Probably approved for a known candidate whose future advisor, or someone else on faculty, is a strong supporter of them; but not automatic for a barely-known person.

On the flip side, admission into the research group of a specific advisor may not be deferrable, except in cases of logistical need (COVID, visas, etc.) That research group may well be expecting someone to start as of a specific date, and not have the flexibility to take more or less people in that and subsequent years. And competitive internal funding may likewise not be deferrable.

It depends on the school. Many (but not all) will let you defer by 1 semester. Some will let you defer by 1 year. More than 1 year would be rare but maybe not absolutely impossible. Just ask the school in question about their policies

You should approach the professor you are applying to, or at least admissions department in your university with that question. Usually the competition is quite high for PhD programs, so in their place, I would ask you to reapply.

However, PhD program may be quite personalized, and they can indeed allow you to join without reapplication. It could also be the case that you can do both at the same time, especially if your work and prospect thesis are related: there are plenty of examples of people who managed to do that.

You may have to reapply as they may give the opportunity to someone else.

Receiving an offer of a place is often a time limited option as they may need the post filled for various reasons that they have.

You need to check with the university that made the offer if they will consider keeping the post open for you.