How do I remove a trailing comma?

I have a snippet of code that is outputting tags for me. I added a comma and space after the name of each tag.
The problem is I do NOT want a comma after the last tag like this:
TAGS: lorum, ipsum, dolores,

What do I add (and where) to get the comma after dolores to go away?
Thank you!!
Liz

Here’s my code:

$tags = get_tags();
$html = '<div class="post_tags">';

echo "<h4> tags: </h4>";

foreach ($tags as $tag){
	$tag_link = get_tag_link($tag->term_id);

	$html .= "<a href='{$tag_link}' title='{$tag->name} Tag' class='{$tag->slug}'>";
	$html .= "{$tag->name}</a>, ";

}
$html .= '</div>';
echo $html;