Someone

<?php
session_start();
if(isset($_GET['kw'] && !empty($_GET['kw'])) {
     $kw = str_replace('/%20/',' ',$_GET['kw']);
     $k1w = urldecode($_GET['kw']); 
} else {
     $k1w = NULL;
}

if(strlen($k1w) < 6) {
  $k1w = "default keyword";
}

echo $k1w; //$k1w is now equal to your keyword if its longer then 6 chars

$_SESSION['keyword'] = $k1w;
?>