$q_tags = '';
$q_tags_arr = array();
foreach($tags as $tag) {
        if(empty($q_tags)) $q_tags .= "tag LIKE ?";
        else $q_tags .= " OR tag LIKE ?";
        $q_tags_arr[] = '%'.$tag.'%';
}

$q_tags_arr[] = $yourValidatedUserInput;
$relatedStmt = $db->prepare(
        'SELECT * FROM items WHERE ('.$q_tags.') AND id <> ? ORDER BY id DESC LIMIT 20'
        );

$relatedStmt->execute(
        $q_tags_arr
);