php variable gets null at the end of the script?

php variable gets null at the end of the script?

I have a problem that I really can't figure out. basically I'm working on
a blog and I have a variable from a cookie that returns a post id. the
problem is that at the end of the script, when I enter an if(isset)
statement, the variable seems to be null and I can't understand the reason
behind it. it won't add anything to the database and it won't redirect to
the post id. here's the script:
<?php
//display the post
$post_id = $_GET['post_id'];
$query = mysql_query("SELECT * FROM posts WHERE `post_id`='". $post_id.
"';") or die(mysql_error());
if(mysql_num_rows($query) != 0)
{
$currentrow = mysql_fetch_array($query);
echo $currentrow['title']. "<br>". $currentrow['text'];
}
else
{
echo "that post does not exist.";
}
?>
</div>
<div id="comments">
<br>
<h3>Comments</h3>
<br>
<?php
//display the comments
$comments = mysql_query("SELECT * FROM `comments` JOIN `posts`
ON(posts.post_id=comments.post_id) JOIN `users`
ON(users.user_id=comments.user_id) WHERE posts.post_id=". "'".
$_GET['post_id']. "'". ";")or die(mysql_error());;
while($currentRow = mysql_fetch_array($comments))
{
echo $currentRow['ctext']. "<br>";
echo "posted by <a href='/templates/profile.php?name=".
$currentRow['name']. "'>". $currentRow['name']. "</a> at ".
$currentRow['cdate']. " -- ". $currentRow['ctime']. "<br><br>";
}
?>
</div>
<Form Name ="submitform" Method ="POST" ACTION = "post.php">
<div id="commentbox">
<textarea name="textarea1" rows="10" cols="30">
<?php
echo $post_id;
?>
</textarea>
<br>
<input type="submit" name="submitcomment" value="Submit"><br>
</div>
</Form>
<?php
if(isset($_POST['submitcomment']))
{
$comment = $_POST['textarea1'];
$user_id = $_COOKIE['userid'];
mysql_query("INSERT INTO comments(`ctext`, `cdate`, `ctime`,
`user_id`, `post_id`) VALUES('$comment', 'CURDATE()', 'CURTIME()',
'$user_id', '$post_id')") or die(mysql_error());
header('Location: /post.php?post_id='. $post_id);
}
?>
</body>
</html>
as you can see, I'm echoing that variable in the textarea1 just before the
if statement and it returns the correct value, but at the end it's null.
thanks in advance.