The PhD degrees as research training dates to 1810, with the foundation of Universität zu Berlin (today Humboldt-Universität zu Berlin or Humboldt University of Berlin) as the first modern research university, offering the first modern-style research PhDs:
These early research PhDs were short compared to modern ones - students could finish in 1 year. Generally, students had to pass an oral exam (or a series of oral exams) and write a dissertation. Similar PhD programs exist today, but are longer than 1 year.
PhD degrees are older than that, but they were quite different - for a start, not intended for research training. A Medieval PhD was often a synonym for an MA, with sometimes the two explicitly combined in a degree like Magister Artium et Doctor Philosophiæ. In this system, university students would first do a general degree ("Arts"), which would be an MA or PhD or similar, and they might then go on to an advanced degree in Theology, Medicine, or Law (aiming for a doctorate in one of those three fields). For more on that, see my past answer linked above.
For more on the early versions on the modern PhD, see:
Actually medieval PhDs are even more diverse. Not only were they sometimes identical to M.A. but they were sometimes awarded simply after attending.
The first PhD diploma in the Holy Roman Empire was awarded in 1359, the first dateable PhD was awarded in Bologna in 1219 (see here http://sundoc.bibliothek.uni-halle.de/diss-online/06/06H046/prom.pdf - unluckily in German). The first PhDs could be obtained in law (iuris), medicine and theology. The artes liberales originally were not eligible for a PhD (hence the M.A.) but that soon changed.