<?php
$n = array (1, 2, 3, 5, 6, 7, 10, 12);
sort ($n);   // If necessary.
$i = 0;
while ($i < count ($n))
  {
    if ($i != 0)
      print (", ");
    $rangestart = $i;
    print ($n [$i++]);
    while ($i < count ($n) && $n [$i] == $n [$i - 1] + 1)
      $i++;
    if ($i > $rangestart + 1)
      print (" - " . $n [$i - 1]);
  }