P40 + 3 NaOH + 3 H2O 3 Na(H2PIO2) + P-IIIH3

This is an oxidation-reduction (redox) reaction:

3 P0 - 3 e- 3 PI (oxidation)

P0 + 3 e- P-III (reduction)

P4 is both an oxidizing and a reducing agent (disproportionation).