Sets the parent of this entity, making it move with its parent.
If an Entity is solid, it probably has a PhysObj. The Entity:Spawn method creates a static PhysObj for a few classes such as solid prop_dynamic, which will never move without the entity it belongs to. Otherwise it creates a dynamic PhysObj, which will fall on the ground when pushed or when the parent moves! Entity:PhysicsInit always creates a dynamic PhysObj whatever the class is. One solution for children with a dynamic PhysObj is to destroy it with Entity:PhysicsDestroy() and make sure to avoid Players colliding with them with Entity:PhysicsDestroy( COLLISION_GROUP_WEAPON ), otherwise Players can get stuck when touching the child.