What is the phylum of cactus plants?

The term that's synonymous with phylum, in the plant world, is division. So the phylum, or preferentially the division, of cactus plants is Magnoliophyta. It's the division for angiOSperms, which also are called flowering plants. Flowering plants may develop from an embryo that has one or two leaves. A cactus is a dicotyledon, because it has two embryonic leaves.