<?php

// In functions.php

function elbone_seo_tags() {
    $tags = get_the_tags();
    $tag_words = array();
    foreach($tags as $tag) {
         $tag_words[] = $tag->name;
    }
    return implode($tag_words, ", ");
    
?>

<!-- In header.php -->
<meta name="keywords" content="<?php echo elbone_seo_tags(); ?>" />