Phosphates are a carbonated beverage made to order for you.
They used to be widespread before the advent of bottled carbonated soda pop, but now are mostly available in kosher restaurants as a dairy-free alternative to ice cream floats or sodas, particularly in Chicago, U.S.
Phosphates are made from carbonated water (not soda water), phosphoric acid, and a flavoured syrup, usually coloured. Phosphoric acid enhances the fizz, and adds a bit of tartness.
The carbonated water comes from a soda fountain tap. Other pumps at the soda fountain offer the different flavours of syrup. In its heyday, flavours would include cherry, chocolate, lime and vanilla. You could also ask for flavours to be mixed: for instance, equal parts of cherry and lime gave you a cherry-lime phosphate. In some areas, one made with lime-flavoured and coloured syrup was called a “Green River.”
A soda glass would be filled about 3/4 full of carbonated water, and a few drops of phosphoric acid added from a bottle. The flavouring was stirred in using a spoon, and the drink was topped up with ice.