A PHI file is a text based representation of the figure hierarchy. Here is an example:-

objFile G:Current ProjectRuntimeGeometriesPhilCFull Assembly
002.obj

1 base xyz<br></br>
 2 lowerArms xyz<br></br>
  3 tray xyz<br></br>
 2 upperArms xyz<br></br>

Each line defines where a body part is in the figure hierarchy.

[hierarchy level] [body part name] [rotation order]

Indentation is not important but it makes it easier for humans to read.

Rotation order is important, the first must be axial to the body part. For a standard human in the T pose the legs and torso will have a first rotation of Y, the arms will be X

Best to pick up Roy Riggs PHI Builder from
http://www.royriggs.com/poser.html

With this you can use it to open your OBJ file then edit the hierarchy drag and drop style.