Dehydration of carboxylic acids with phosphoric acid
The $\ce{-OH}$ part will not get protonated, since its lone pair is in resonance with the carbonyl part of the $\ce{-COOH}$ group. Instead, the lone pair of the carbonyl part will give its lone pair to $\ce{H+}$. 

Something like this :-



It may look insignificant because the product is same, but this is the correct mechanism.