Just finished 3 months of PHUL... one of my favorite programs. Really allows you to scratch your volume itch.

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You should go up in weight when you can reach the maximum amount of recommended reps for every single set. So, for example, if the program says to do an exercise for 3 sets of 8-12 reps, you should find a weight that allows you to do AT LEAST 8 reps for all 3 sets. Eventually, you'll get stronger and be able to do more reps each set. Once you can complete 12 reps for every set, increase the weight. Presumably, with the new higher weight, the amount of reps you can do in a single set will decrease. The process starts over. Once you get that new higher weight to 12 reps for all 3 sets, increase again, etc...