$x = 4;

if (isset($_POST['gal_tab_x'])) { // formular byl poslan, menime hodnotu

  $x = (int)$_POST['gal_tab_x'];

  if ($x >= 2 and $x <= 6) { // je-li platna

    setcookie('tabulka_x', $x);

  }

} elseif (isset($_COOKIE['tabulka_x'])) {

  $x = (int)$_COOKIE['tabulka_x'];

  if ($x < 2 or $x > 6) { // neni-li platna

    $x = 4;

  }

}

$tx = 4;
if (isset($_COOKIE['tabulka_x'])) {

  $tx = (int)$_COOKIE['tabulka_x'];

  if ($tx < 2 or $tx > 6) { // neni-li platna

    $tx = 4;

  }

}

<?php

$P = 24;

$k = 8;

$vysl = array('a','b','c','d','e','f','g','h','i','j','k','l','m');

$t = '<table border=1>';

$length = count($vysl);

$poli = max($P, $k); 

for ($i = 0; $i < $poli; $i += $k) {

   $t .= '<tr>';

   for ($j = $i; $j < $i + $k; ++$j) {

      if ($j < $length) {

         $field = $vysl[$j];

      } else {

         $field = ' ';

      }

      $t .= '<td>'.$field.'</td>';

   }

   $t .= '</tr>';

}

$t .= '</table>';

echo $t;

?>