Multiple Select with Explode: only returns the word “Array”

I have created a multiple select box so that users can select categories to exclude. When the page initially loads I see my default values pre-selected. However when I select new values and save… I only see the word “Array” in my source code.

<select class="amultiple" id="tt_cat_exclude"  name="tt_cat_exclude[]" multiple="multiple" size="8">
			<?php
				global $options;
					foreach ($options as $value) {
						if (get_settings( $value['id'] ) === FALSE) { $$value['id'] = $value['std']; } else { $$value['id'] = get_settings( $value['id'] );
						}
					}
					$categories = &get_categories('type=post&orderby=name&hide_empty=1');
					if ($categories) {
						$ex_cat = explode(',', $tt_cat_exclude);

						foreach ($categories as $category) {
							$selected = (in_array($category->cat_ID, $ex_cat)) ? ' selected="selected"' : '';
							echo '<option value="' . $category->cat_ID . '"' . $selected . '>' . $category->cat_name . '</option>' . "\n";
						}
					}
?>
</select>
For testing purposes, print variables: <?php echo $tt_cat_exclude; ?> & <?php echo $ex_cat; ?>