PHP: Session stored in database not always updated

When storing sessions in a database, I have noticed that updating a session does not necessarily invoke the write function that was set in session_set_save_handler(). This problem seems to be solved when you call session_write_close() at the end of the script.

Am I missing something or is this indeed what needs to be done when storing sessions in a database using session_set_save_handler()?

The code that I am currently using.

// EXECUTE AT START OF SCRIPT
$session = new Session();
session_start();

Session Class

public function __construct() {
    $database       = Database::instance();
    $this->dbh      = $database->dbh();
    $this->lifetime = get_cfg_var('session.gc_maxlifetime');

    session_set_save_handler(
        array(&$this, 'open'),
        array(&$this, 'close'),
        array(&$this, 'read'),
        array(&$this, 'write'),
        array(&$this, 'destroy'),
        array(&$this, 'clean')
    );
}

// SESSION HANDLER METHODS
function open($savePath, $sessionName) {
    return true;
}

function close() {
    return true;
}

function read($sessionUuid) {
    logMessage('READ SESSION DATA > ' . $sessionUuid);
    $data = array();
    $time = time();

    $sth = $this->dbh->prepare("SELECT sessionData FROM phpSession WHERE sessionUuid = :sessionUuid AND expires > " . $time);
    $sth->setFetchMode(PDO::FETCH_ASSOC);

    try {
        $sth->execute(array('sessionUuid' => $sessionUuid));

        if ($sth->rowCount() == 1) {
            $row = $sth->fetch();
            $data = $row['sessionData'];
        }

    } catch (PDOException $exception) {
        // HANDLE EXCEPTION
    }

    return $data;
}

function write($sessionUuid, $sessionData) {
    logMessage('WRITE SESSION DATA > ' . $sessionUuid);
    $time = time() + $this->lifetime;

    $sth1 = $this->dbh->prepare("SELECT sessionUuid FROM phpSession WHERE sessionUuid = :sessionUuid");

    try {
        $sth1->execute(array('sessionUuid' => $sessionUuid));

        $query  = '';
        $data   = array(
            'sessionUuid'   => $sessionUuid,
            'sessionData'   => $sessionData,
            'expires'       => $time
        );

        if ($sth1->rowCount() == 1) {
            // UPDATE
            $query = "UPDATE phpSession SET sessionUuid = :sessionUuid, sessionData = :sessionData, expires = :expires";

        } else {
            // INSERT
            $query = "INSERT INTO phpSession (sessionUuid, sessionData, expires) VALUES (:sessionUuid, :sessionData, :expires)";
        }

        $sth2 = $this->dbh->prepare($query);

        try {
            $sth2->execute($data);

        } catch (PDOException $exception) {
            // HANDLE EXCEPTION
        }

    } catch (PDOException $exception) {
        // HANDLE EXCEPTION
    }

    return true;
}

function destroy($sessionUuid) {
    $sth = $this->dbh->prepare("DELETE FROM phpSession WHERE 'sessionUuid' = :sessionUuid");

    try {
        $sth->execute(array('sessionUuid' => $sessionUuid));

    } catch (PDOException $exception) {
        // HANDLE EXCEPTION
    }

    return true;
}

function clean() {
    $sth = $this->dbh->prepare("DELETE FROM phpSession WHERE 'expires' < UNIX_TIMESTAMP()");

    try {
        $sth->execute();

    } catch (PDOException $exception) {
        // HANDLE EXCEPTION
    }

    return true;
}