It depends on the victim and the circumstances.
Consider the following:
- How intelligent is the target of the spell?
- How out-of-place is the circumstance? (cyclone indoors? earthquake on a ship?)
- How long has the illusion been in effect? (My axe went straight through it?!)
For your Orc situation, they are reasoning creatures if not particularly well-educated.
Initially, the Orc will probably be surprised and take some psychic damage from the wolf.
On its first turn under the spell effects, the Orc will probably attack the illusion. This is probably not the first time the Orc has fought a wild animal, and it should know that hitting a wolf isn't the most difficult thing in the world. It may also recognize that, without a pack (strange!) to back it up, the wolf should be no problem at all. This attack will fail to kill the wolf, because it isn't there (or, if he rolls really horribly, this might take a round or two to reach the conclusion that the wolf isn't normal).
The next turn, the Orc might spend trying to disbelieve the illusion. That is, to say, after the target realizes that there's something off about the spell's effect, it may try to investigate its way through the illusion.
Regardless of success or failure, on the next turn the Orc will probably prioritize a different threat: The caster (druid/ranger most likely, then arcane casters) who is most likely causing this unpleasant circumstance, or something doing more than 1d6 damage to it. These priorities will, however, take into account how much damage the Orc will take trying to reach that target.
--Considering the size of the illusion that you can make with Phantasmal Force (10x10), depending on how the caster narrates the illusion, you may also have the Orc react in a way other than receiving damage - i.e. fall prone as it tries to defend itself, or fall over an edge as it panics, or run in terror, fall unconscious from strangulation...
Illusion really leaves a lot of room for flavor, flair and interpretation.