<?php
$ns = [ 691.605, 691.615, 691.625, 691.635, 691.645, 691.655, 691.665, 691.675, 691.685, 691.695, 17.565, 19.875 ];

foreach ($ns as $n) {
  $s = sprintf("%.2f", $n);
  $l = sprintf("%.20f", $n);
  print "$n: $s\t$l\n";
}