What is the mechanism for the creation of P(CH2OH)4+ cation? I know that the initial reactants are phosphine and menthanal in an acid ($\ce{H+ A-}$) and that the first step in the reaction is the HOMO of phosphine (the lone pairs) and the LUMO of methanal (the π* orbital). 

Then, the electrons that just moved to the oxygen atom react with $\ce{H3O+}$, so the oxygen is protonated. I'm not sure what follows, though.
The acid serves as a catalyst to protonate the formaldehyde oxygen.  This makes the carbonyl carbon more electrophilic, a more reactive target for the weakly nucleophilic lone pair of electrons on phosphorous.  Here is a diagram of the basic reaction mechanism.



The proton transfer is thought to be intramolecular according to this link.