Need to add text in PHP function

I know next to nothing about PHP, but usually I get by with some editing. But not today!

I have this code:

$small_prev_image = post_thumb($post->ID, array('size'=> array('M','_external'),
													 'wh' => $k_option['custom']['imgSize']['M'],
													 'img_attr' => array('class'=>'item_small')	

‘_external’ is a URL. but I need to add some code before and after the URL, but can’t figure out how to do this… Can anyone help me out?

Yes, it gets displayed like this:

echo "<div class='gallery_entry gallery_entry_$loopcount $last'>";
	echo "<div class='gallery_inner'>";
	echo "<a class='preloading gallery_image' href='".$imagelink."'>";
	if($featured == 'yes') echo "<span class='featured_entry'></span>";
	echo $small_prev_image;
	echo $big_prev_image;
	echo "</a>";
	echo "<span class='comment_link'>";
	comments_popup_link(__('0','expose'), __('1','expose'), __('%','expose'));
	echo "</span>";
	if(function_exists('the_ratings')) the_ratings();		
	echo "<div class='gallery_excerpt'>";
	echo get_the_excerpt();
	echo "</div>";
	echo "</div>";
	echo "<h3><a href='".$titlelink."'>".get_the_title()."</a></h3>";
	echo "</div>";

the _external will display the URL. I want to use a thumnail service. so if _external is, it will need to become something like

What code you want to append before and after the URL and what is the meaning of adding code before and after a URL? Can you elaborate your requirement with sample value?