Reaction of white phosphorus with aqueous sodium hydroxide
Actually, there are two reaction involving:


$$\ce{P4 + 3NaOH + 3H2O -&gt; PH3 + 3NaH2PO2}$$



  White phosphorus react with sodium hydroxide and water to produce
  phosphine and sodium hypophosphite . Sodium hydroxide - cold,
  concentrated solution. This reaction takes place slowly. (Chemiday 1)


The O.S. of phosphorus in sodium hypophosphite is calculated in @Sayantan.G's answer.


$$\ce{P4 + 8NaOH + 4H2O -&gt;[\Delta]  4Na2HPO3 + 6H2}$$



  White phosphorus react with sodium hydroxide and water to produce
  disodium hydrogen phosphite and hydrogen. Sodium hydroxide -
  concentrated solution. The reaction takes place in a boiling solution.(Chemiday 2)


The O.S. of phosphorus in disodium hydrogen phosphite is:

$$\ce{ +2*1 + 1 + x + 3*(-2) = 0}$$

$$\ce{ x = +3}$$