What is the mechanism for the creation of P(CH2OH)4+ cation?
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.