Eeplace text with category description

Hi,

I want to replace in post, a text (ex. ‘banner’) with the category description.

For category description I use this code:

$categories = get_the_category();
  	foreach($categories as $key => $category) {
    	$url = get_term_link((int)$category->term_id,'category');
    	$categories[$key] =
      	"<dd>{$category->category_description}</dd>";
  	}
  echo "<dl>\n" . implode("\n",$categories) . "\n</dl>";

How can I do that?