Catalyzes the isomerization between 3-hexulose 6-phosphate and fructose 6-phosphate. D-arabino-hex-3-ulose 6-phosphate = beta-D-fructose 6-phosphate One-carbon metabolism; formaldehyde assimilation via RuMP pathway; D-fructose 6-phosphate from D-ribulose 5-phosphate and formaldehyde: step 2/2. By methanol or methylamine. Belongs to the SIS family. PHI subfamily.