Преобразовать строку в букву на PHP

Допустим, 4 числа: 1 2 3 4

<?php
function simple_hash ($s) {
  $c = [1, 2, 3, 4];
  $a = unpack ('C*', $s);
  return $c [array_reduce ($a, function ($a, $n) {return $a += ceil ($n / 256);}, 0) % count ($c)];
}
echo '<pre>' . print_r (simple_hash ('Hello world!'), true);

    <?php
function simple_hash ($s) {
  $c = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16];
  $a = unpack ('C*', $s);
  return $c [array_reduce ($a, function ($a, $n) {return $a += ceil ($n / 256);}, 0) % count ($c)];
}

while ($i < 17) {
    $i ++;
    $arr[simple_hash ($i)]++;
}
print_r($arr);

Результат:

Array
(
    [2] => 9
    [3] => 8
)