PDAF would be using the specific AF sensors that are present on the camera.

The number and coverage area of these sensors increase in relation to the price of the product
Because of Sony's SLT technology, they are in a fairly unique situation to offer both PDAF (thru AF sensors) and CDAF (through the image sensor itself) at the same time. This opens up the possibility of 'clashing' though.
On my Nikon, I have face detection but only in LiveView (CDAF), so there's no issue about getting mixed up.

My suggestion would be to forget about face detection because it's too gimmicky