Interesting. Could fungi photosynthesize? I thought not. Unless the earth was covered as a marshland, it would be unlikely mushrooms covered earth. It is likely they are popular around coastal area.
No known fungus does--though there are "radiotrophic" fungus, that use gamma radiation as an energy source.
However, fungus are found with photosynthetic symbionts. Lichen, for example, are a combination of fungus and algae (or similar). It's possible the Prototaxites were giant lichen, which would allow them to live in non-fungus-friendly locales, rather like a tree.