I've also been to 2 different ones. They both started out with use standing in a group, the staff introducing themselves and telling us what it's like to work for P@H and the sort of training you can do, then we got split into smaller groups for a "group task". The first one I went to (I never ended up getting the job), the group task was to design a fish tank so we'd chose a tank in store and then find all the bits and pieces we wanted to put in and add those plus a list of the fish we wanted. We had to decide on a target audience for the tank and market it towards them and then we had to "sell" it to the staff.
The second one I went to, the group task was a lot more amusing. We were each given 5 newspapers and some tape and had 25 minutes to build a tower as tall as possible without sticking it to the floor or anything else to help hold it up
My group came second
After the group assessment thing, they get you all back together and then send you off one at a time with a member of staff to do a bit of an informal thing which is basically asking you when you're available, where you see yourself in 3 years, just basic stuff really and you can ask any questions you want. I always ask about career development and opportunities to move up the ladder as it shows ambition and stuff. When you've finished this bit you can leave and you usually find out within a few days whether you get through to the next stage (work trial) or whether you've been unsuccessful.
I'm waiting to hear back from my second one at the moment but the manager from my local is on holiday this week so I won't hear anything until she gets back next week.
It's worth remembering, as racinghamster said, that they are retail first so they value retail experience over animal experience. I guess it's easier for them to put you through their animal training rather than having to train you how to do retail. Most important thing though, be yourself and have fun.