Catalyzes the isomerization between 3-hexulose 6-phosphate and fructose 6-phosphate. D-arabino-hex-3-ulose 6-phosphate = beta-D-fructose 6-phosphate Carbohydrate biosynthesis; D-ribose 5-phosphate biosynthesis. Homotetramer. Belongs to the SIS family. PHI subfamily.