Pholcodine is an opioid so you need to avoid it in my opinion. Don't beat yourself up about it, though and as you have only just started theres no real lost time in any case.
Codeine is used as a cough suppressant so it's not unusual to develop the opposite effect during withdrawal. This will go away on it's own in a few days, so don't be too alarmed.
Petroglph gives good advice, however if the full Thomas Recipe seems a bit much I'd suggest getting some Imodium or equivalent, Valerian, plenty of fluids and mild exercise. All these things will help reduce the severity of any withdrawal symptoms you experience.
Hope you're doing well! Codeine is a strong drug and it's a struggle but you can fight and win this battle. It's not easy and there's ups and downs but it's soooo worth it in the long run!
I can't answer that question for ya. But as far as addiction goes I don't know that it matters what a person't drug of choice is. Most of addiction is mental, It doesn't seem like it when new to getting clean. What I would like to offer first is welcome to the forum, there are many people just like you in the same situation, and it seems to help to have someone that knows to share this with. I also want to encourage you to read the "thomas recipe" at the bottom right on this page. Your body and mind are going to want good nutrition, clean water, and exercise to return it to the way it was made. Please don't get discouraged, sobriety is possible for all of us.