<input id="input" value="100000000000000000000000000000000000000000000000000000000000000000000000000000">



<script>document.getElementById("input").value = new Intl.NumberFormat('ru-RU').format( document.getElementById("input").value );</script>

<?php

$monsters = [[1, 1000, ], [1000, 1000000, k]];

foreach ($monsters as $monster)

  if ($i >= $monster[0] and $i < $monster[1]) {

    $i = number_format($i, 0, ‘’, ‘.’);

    $i = round($i, 1) . $monster[2];

    break;

  }

?>