PHP Fatal error: Uncaught PDOException: SQLSTATE[HY093]
Maybe try something like:
$query = "SELECT session_id FROM user_sessions WHERE session_id = :session_id AND userid = :userid";
$stmt = $this->db->prepare($query);
$stmt->execute(array(':userid' => $userid, ':sessionid' => $session_id));
The problem I see is that you query does not contain any params, however, you pass some into execute method.
Check docs, especially :
$sth = $dbh->prepare('SELECT name, colour, calories FROM fruit WHERE calories < :calories AND colour = :colour');
$sth->execute(array('calories' => $calories, 'colour' => $colour));