<?php

function f($n) {
    static $result = [];

    if (isset($result[$n])) {
        return $result[$n];
    }

    if ($n <= 2) {
        $r = $n;
    }  else {
        $r = f($n - 1) + f($n - 2);
    }
    $result[$n] = $r;
    return $r;
}

echo f(40);