What happens when you mix Disodium Phosphate with Sodium Hyrdroxide?

it makes a solution of sodium hydroxide, also called "caustic soda" or "lye"

in dissolving, heat is produced - a lot of heat.

always add sodium hydroxide to water --- NEVER water to sodium hydroxide (it can be explosive in mixing due to the heat)

It should also be noted that the water should always be COLD. Adding sodium hydroxide to hot water will increase the exothermic reaction and result in dangerous spattering.