Generate meta keywords from post tags

get_meta_keywords.php

<?php

/**
 * Generate meta keywords from post tags
 * 
 * @uses wp_get_post_tags
 * @return string keywords
 *
 * Usage: 
 * <meta name="keywords" content="<?php echo get_meta_keywords(); ?>">
 */
function get_meta_keywords() {
    global $post;
    $keywords = wp_get_post_tags( $post->ID );

    $total = count( $keywords );
    $count = 1;
    $keys  = '';

    foreach ( $keywords as $keyword ) { 
        $keys .= $keyword->name;
        if ( $count != $total ) {
             $keys .= ', ';
        } 
        $count++;
    }
 
    return $keys;
}