task 15

<?php
$signature = [2,13,0,12,20,4,5,4,15,9,15,3,17,9,15,10,6,12];
$n = 49;

function xbonacci( $signature, $n){
$l = count ($signature);
if ($n == 0) {
    return ($arr=[]);
    }
else {
        while (count ($signature) < $n) {
            $arr = array_slice ($signature, -$l);
            $s = array_sum ($arr);
            $signature[] = $s;
            }
    $output = array_slice($signature, 0, $n);
    return ($output);
    }
}