replacing the second variable

[gerkintrigg]

I might be totally mad, but can someone please tell me why this script is not replacing the string at all in the $_SESSION['page'] variable, please!?

<?php 
// start the session 
session_start(); 
$root='../';
header("Cache-control: private"); //IE 6 Fix 
$page = $_SESSION['page'];

$original_word=$_POST['original_word']; // id number

$instance=($_POST['instance']-1); // instance counter
echo $instance;
if(empty($_POST['manual_replacement'])){
$replacement_word=$_POST['replacement_word'];
}
else{
$replacement_word=$_POST['manual_replacement'];
}

if($_POST['instance_checkbox']=='y'){
$instance='';
}
#still need to work out replacing same case:
if($replacement_word!=''){
$pattern='~<span id=\"'.$original_word.'_nst_id_'.$instance.'(.+?)">(.+?)<\/span>~';
$page = preg_replace($pattern, $replacement_word, $page);
}
$_SESSION['page']=$page;
$go='outputs/highlighted.php';
header("Location: ".$go);
?>