A professor may have a small influence on the admission of a candidate, but the way admissions decisions are made will vary across departments.
I think, in general, if an applicant is already known to a member of the department and they would be happy to take them on as student, that applicant's chance of an offer of a PhD place is increased. However, I would still expect that a formal application, satisfactory references and an interview with other faculty members would be required before an offer was made concrete.
I don't know what field you are in, but in mine (physics), funding for a PhD generally comes either from a research council (i.e. the government) or from the university itself. A single professor will have almost no influence on where that money goes. Funding decisions may be made after admission decisions and completely separately to that process, unless the candidates are applying for a specific funded project.
Speaking from my own experience, I was made an informal offer of a place at a university where I know one of the professors very well. However, funding in that department was scarce and the money ultimately went to another, stronger candidate (as decided by a committee that my supervisor had no influence on). Consequently, I interviewed elsewhere, and was fortunate to be made a fully funded offer at another university.
To summarise, if a professor really wants you on board, they may be able to offer you admission, but the funding decision is likely to be made by someone else. (This of course changes if the professor has some grant money of their own by which they can fund a student, or if the student is self-funded.)