To say it more precisely:
There is no problem placing a physics object with vec_for_vertex - once, before the physics starts, but you can't update it _while_ using the physics properties, because fixing to a certain place contradicts physical properties. The fixing has to happen by a physical property, and that's where the hinges come in.
From the manual:
When connecting two objects by calling phcon_add, make sure that they are already in the right position and don't penetrate each other- if they are meant to penetrate a little (e.g. wheels inside the car body's bounding box), set both objects to the same group ID using phent_setgroup. The following list provides an overview of currently available constraints and their required parameters.
PH_HINGE
Type:
hinge (10K)
Description : A hinge constrains two entitites to have a common axis of rotation and to stay a fixed distance from this axis. As depicted in the image above, you need to specify this rotation axis and an anchor point on said axis. The distance from each entity to the axis is determined when you first create the constraint. Therefore make sure to place the entities in the right position, before adding the hinge. The initial orientation is also taken as an angle value of zero degrees. Thus if both entities are rotated by 90 degrees with respect to each other and you set the angle limit to (0,0) they will remain in the initial orientation.
