[BLOCK] Combobox with categories

I've written a block with a combo with categories. The user selects a category and the block follows with displaying the category listing.

I'm beginning to learn writing something in php, so everything is simple.
The problem I'm having is that it does not saves the selected category. I prefer this category in a session variable.

You can download the block block here: http://forum.writingcenter4.info/index.php?action=mgallery;sa=item;id=11335
You can see the block in action here: http://ficdom.net/index.php

The content of the categorycombo is:

<?php
$querystring = "";
if(!defined("_CHARSET")) exit( );

$cookiecatid = $_SESSION['cookiecatid'];

$content = "<select name="categorycombo1" onChange="document.location = ' http://ficdom.net/browse.php?categorychange=1&type=categories&catid='  + this.options[this.selectedIndex].value;">";

$query = dbquery("SELECT * FROM `ficdom_fanfiction_categories` WHERE `parentcatid` = -1 and`numitems`>0 ORDER BY `category`");
while($categories = dbassoc($query))
{
$id = $categories['catid'];
$name = htmlspecialchars($categories['category']);
$itemcount = $categories['numitems'];
$displayname = "".$name." [$itemcount]";
if($id == "$cookiecatid"){
$content .= "<option value=$id selected>$displayname</option>";
}
else{
$content .= "<option value=$id>$displayname</option>";
}
}
$content .= "</select>";
?>

You must add {categorycombo_title} and {categorycombo_content} in your skin after you copied the content of the categorycombo.zip in the block directory of efiction.

As you can see on the session statement

$cookiecatid = $_SESSION['cookiecatid'];

that I tried to save the catid into a session in browse.php, but that did not work very well. It saves the catid correctly, after the block was already loaded.
Ideally, the session would be updated in the block itself:

$content = "<select name="categorycombo1" onChange="document.location = ' http://ficdom.net/browse.php?categorychange=1&type=categories&catid='  + this.options[this.selectedIndex].value;">";

But how to do that, no idea. Can anyone help me with this?