Can just ANYONE call themselves a Doctor?

Now a days, with all these “online schools” offering PhD’s, does this mean that just anyone can call themselves a Doctor? Or attach Dr. to their names? For example, if I graduated from University of Phoenix (or any of those other quacky schools) with a PhD, could I now call myself Dr. Angela just by typing it up on a business card? Or are there certain requirements to be able to call yourself a doctor? If a college professor studied years and years to get a PhD, and another college professor got a “PhD” online, doesn’t that just seem unfair?