Loop to add previous four numbers for each number starting from 4 and then assign first , second , third to second , third , fourth and curr to fourth respectively
for ( $ i = 4 ; $ i <= $ n ; $ i ++ ) { $ curr = $ first + $ second + $ third + $ fourth ; $ first = $ second ; $ second = $ third ; $ third = $ fourth ; $ fourth = $ curr ; } echo $ curr ; } }