vendor/symfony/http-foundation/Session/Storage/Handler/PdoSessionHandler.php

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<?php

/*
 * This file is part of the Symfony package.
 *
 * (c) Fabien Potencier <fabien@symfony.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Symfony\Component\HttpFoundation\Session\Storage\Handler;

/**
 * Session handler using a PDO connection to read and write data.
 *
 * It works with MySQL, PostgreSQL, Oracle, SQL Server and SQLite and implements
 * different locking strategies to handle concurrent access to the same session.
 * Locking is necessary to prevent loss of data due to race conditions and to keep
 * the session data consistent between read() and write(). With locking, requests
 * for the same session will wait until the other one finished writing. For this
 * reason it's best practice to close a session as early as possible to improve
 * concurrency. PHPs internal files session handler also implements locking.
 *
 * Attention: Since SQLite does not support row level locks but locks the whole database,
 * it means only one session can be accessed at a time. Even different sessions would wait
 * for another to finish. So saving session in SQLite should only be considered for
 * development or prototypes.
 *
 * Session data is a binary string that can contain non-printable characters like the null byte.
 * For this reason it must be saved in a binary column in the database like BLOB in MySQL.
 * Saving it in a character column could corrupt the data. You can use createTable()
 * to initialize a correctly defined table.
 *
 * @see https://php.net/sessionhandlerinterface
 *
 * @author Fabien Potencier <fabien@symfony.com>
 * @author Michael Williams <michael.williams@funsational.com>
 * @author Tobias Schultze <http://tobion.de>
 */
class PdoSessionHandler extends AbstractSessionHandler
{
    /**
     * No locking is done. This means sessions are prone to loss of data due to
     * race conditions of concurrent requests to the same session. The last session
     * write will win in this case. It might be useful when you implement your own
     * logic to deal with this like an optimistic approach.
     */
    public const LOCK_NONE = 0;

    /**
     * Creates an application-level lock on a session. The disadvantage is that the
     * lock is not enforced by the database and thus other, unaware parts of the
     * application could still concurrently modify the session. The advantage is it
     * does not require a transaction.
     * This mode is not available for SQLite and not yet implemented for oci and sqlsrv.
     */
    public const LOCK_ADVISORY = 1;

    /**