尝试提交事务时出现致命错误

Question

以下是使用PDO进行数据库操作的Database类。 我为CRUD操作和启动和停止事务实现了单独的方法。

    //for localhost
    private static $dbName = Config::dbName;
    private static $dbHost = Config::dbHost;
    private static $dbUsername = Config::dbUsername;
    private static $dbUserPassword = Config::dbUserPassword;


    private static $cont = null;

    /*
     * Connect to the database
     */
    public static function connect()
    {
        // One connection through whole application
        if (null == self::$cont) {
            try {
                self::$cont = new PDO(
                    "mysql:host=" . self::$dbHost . ";"
                    . "dbname=" . self::$dbName, self::$dbUsername, self::$dbUserPassword);
                echo '<br/>Connected to Database<br/>';
            } catch (PDOException $e) {
                die($e->getMessage());
            }
        }

        return self::$cont;
    }

    /*
     * Disconnect the database
     */
    public static function disconnect()
    {
        echo '<br/>Disconnected to Database<br/>';
        self::$cont = null;
    }

    /*
      * Begin Trancation
      */
    public function beginTransaction()
    {
        /* Begin a transaction, turning off autocommit */
        self::connect();
        echo '<br/>Begin Transaction<br/>';
        self::$cont->beginTransaction();
    }

    /*
  * Commit Trancation
  */
    public function commitTransaction()
    {
        if (self::$cont = null) {
            throw  new  Exception('First start transaction');
        } else {
            try {
                echo '<br/>Commit Transaction<br/>';
                /* Commit the changes */
                self::$cont->commit();
                /* Database connection is now back in autocommit mode */
            } catch (Exception $e) {
                die('Commit Error ' . $e->getMessage());
            }
        }
    }
/**
     * Prepare Query and Execute
     *
     * @param $query The query string
     * @param $data Array of data to be insert
     * @return retrun connection
     */
    private function execute($query, $params)

    {

        // Connect to the database
        $cont = $this->connect();
        // Setting it to error mode
        $cont->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        $executor = $cont->prepare($query);
        $executor->execute($params);
        return $cont;

    }
/**
     * Insert single record to Database
     *
     * @param $query The query string
     * @param $data Array of data to be insert
     * @return last inserted id
     */
    public function insert($query, $data)
    {
        try {
            $cont = $this->execute($query, $data);
            $result = $cont->lastInsertId();
            echo '<br/>Data Inserted witb Id' . $result . '<br/>';
            return $result;
        } catch (PDOException $e) {
            die($e->getMessage());
        }
    }

现在我试图通过以下方式使用它，它给了我致命错误

require '../class/Database.php';


echo 'Start Transaction<br/>';
$sql = "INSERT INTO pm_bib (bib_key,bib_type,bib_title,param,value) values(?, ?, ?,?,?)";
$db = new Database();
$db->beginTransaction();
echo 'Insert 1 : ok<br/>';
$param = array(1, 1, 1, 1, 1);
$db->insert($sql, $param);
echo 'Insert 2 : error<br/>';
$param = array(1, 2, 2, 2, 2);
$db->insert($sql, $param);
$db->commitTransaction();

echo 'End Transaction<br/>';