As phospholipids are major composition of cell membrane, it posses a hydrophobic part , the lipid , as it is non-polar part it fails to dissolve in water .
At the same time, phosphate group is polar in nature it easily dissolved up water, as it result it forms the hydrophilic part.
So simply the phosphate group forms one layer ( the polar one) while another lipid group forms second layer of bilayer ( the non-polar one).