${\mathrm{P}}_4{\mathrm{O}}_6$ is the anhydride of

An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom.

${\mathrm{P}}_4{\mathrm{O}}_6+6{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{H}}_3{\mathrm{PO}}_3$