With this:
PHP Code:
SDKHook(proj, SDKHook_OnTouchPost, OnArrowTouchPost);
public void OnArrowTouchPost(int entity)
{
// put the damage code here
SDKUnhook(entity, SDKHook_OnTouchPost, OnArrowTouchPost);
AcceptEntityInput(entity, "Kill");
RemoveEdict(entity);
}