After turning on the hot water knob to required output... the knob keeps turning by itself... letting more and more water out... why?

One or two handle valve? What brand is it? Back to you. Tom

One or two handle valve? What brand is it? Back to you. Tom

It is two handle valve and the brand is price phister.

The knob fits over splines on the stem, This is what prevents it from slipping.
It's my guess that the splines on the handle are stripped. Replace the knob. Good luck, Tom

If the knob is tight, take it off and seenif you can tighten the packing nut for the stem. Sometimes they will be loose enough to let the stem turn but not leak.