v2

global $posts, $wpdb;

if ( is_single() ) {
$page_keywords = get_the_tag_list('', ' ', '');
}
elseif (is_home() || is_category() || is_tag() || is_date() || is_search() ) {
foreach ($posts as $post_values) {
$array_id[] = $post_values->ID;
}
$list_id = implode('","', $array_id);
$sql_query = 'SELECT DISTINCT name  '.
'FROM '.$wpdb->term_relationships.' AS rel,'.
$wpdb->term_taxonomy.' AS tax,'.
$wpdb->terms.' AS ter '.
'WHERE tax. taxonomy = "post_tag" '.
'AND rel.object_id IN ("'. $list_id.'") '.
'AND rel.term_taxonomy_id = tax.term_taxonomy_id '.
'AND tax.term_id = ter.term_id ORDER BY tax.count';
$results = $wpdb->get_results($sql_query);
if ($results) {
$page_keywords = implode(' ', $results);
} else { ?>
<script type="text/javascript">
alert ("The $results variable is empty.");
</script>
<?php }
}
if ($page_keywords != '') {
$page_keywords = htmlspecialchars(stripslashes(strip_tags(str_replace("\n", '', $page_keywords))));
echo '<meta name="keywords" content="'.$page_keywords.'">'."\n";'