Your form is using ‘get’ ( $_GET ), but addcat.php is looking for $_POST values. That is never going to work. But beyond that, you’ve got a pretty convoluted thing going on here. You don’t really both the hooked function and addcat.php. You need one or the other, and the hooked function is probably the better idea. Something like:
function addPostToCategoryLauncher() {
global $_GET;
if (isset($_GET['cat']) && isset($_GET['postID')) { // I am assuming that 'cat' is the correct parameter name
wp_set_object_terms( $_GET[postID], $_GET['cat'], 'category' ,true);
}
}
add_action( 'init', 'addPostToCategoryLauncher' );
You will need to send the postID with your form submission.
<form action="<?php bloginfo('url'); ?>/addcat.php" method="get">
<input type="hidden" name="postID" value="<?php echo $post->ID; ?>" ?>
<div>
<?php
$select = wp_dropdown_categories('show_option_none=Select category&show_count=0&orderby=name&echo=0&child_of=28&hide_empty=0');
$select = preg_replace("#<select([^>]*)>#", "<select$1 onchange="return this.form.submit()">", $select);
echo $select;
?>
<noscript><div><input type="submit" value="View" /></div></noscript>
</div>
</form>
These are barebones functions. You need to incorporate some sanity checking for the $_GET values and you really should use nonces as well.
I’ve made some assumptions about how your code works so there could be glitches and there may be a better hook that init but I’d have to look around.
Try that.