External database connection

Oct 13, 2025 by wbrum.org 47 views

[5.x]: session_start(): Unexpected end of serialized data

What happened?

When upgrading from Craft 4 to Craft 5, users may encounter an unexpected error when using both craft.app.session and the cookies plugin in their templates. This error occurs when fetching data from an external database, as described in this article. The error message is:

yii\base\ErrorException: session_start(): Unexpected end of serialized data in /var/www/html/vendor/yiisoft/yii2/web/Session.php:150
Stack trace:
#0 /var/www/html/vendor/craftcms/cms/src/web/ErrorHandler.php(115): yii\base\ErrorHandler->handleError(8, 'session_start()...', '/var/www/html/v...', 150)
#1 [internal function]: craft\web\ErrorHandler->handleError(8, 'session_start()...', '/var/www/html/v...', 150)
#2 /var/www/html/vendor/yiisoft/yii2/web/Session.php(150): session_start()
#3 /var/www/html/vendor/yiisoft/yii2/web/Session.php(750): yii\web\Session->open()
#4 /var/www/html/vendor/craftcms/cms/src/helpers/Session.php(95): yii\web\Session->has('__duration')
#5 /var/www/html/vendor/craftcms/cms/src/web/User.php(528): craft\helpers\Session::has('__duration')
#6 /var/www/html/vendor/yiisoft/yii2/web/User.php(199): craft\web\User->renewAuthStatus()
#7 /var/www/html/vendor/craftcms/cms/src/web/Application.php(455): yii\web\User->getIdentity()
#8 /var/www/html/vendor/craftcms/cms/src/web/Application.php(112): craft\web\Application->debugBootstrap()
#9 /var/www/html/vendor/yiisoft/yii2/base/BaseObject.php(109): craft\web\Application->init()
#10 /var/www/html/vendor/yiisoft/yii2/base/Application.php(204): yii\base\BaseObject->__construct(Array)
#11 [internal function]: yii\base\Application->__construct(Array)
#12 /var/www/html/vendor/yiisoft/yii2/di/Container.php(419): ReflectionClass->newInstanceArgs(Array)
#13 /var/www/html/vendor/yiisoft/yii2/di/Container.php(170): yii\di\Container->build('craft\\web\\Appli...', Array, Array)
#14 /var/www/html/vendor/yiisoft/yii2/BaseYii.php(365): yii\di\Container->get('craft\\web\\Appli...', Array, Array)
#15 /var/www/html/vendor/craftcms/cms/src/Craft.php(71): yii\BaseYii::createObject(Array, Array)
#16 /var/www/html/vendor/craftcms/cms/bootstrap/bootstrap.php(343): Craft::createObject(Array)
#17 /var/www/html/vendor/craftcms/cms/bootstrap/web.php(40): require('/var/www/html/v...')
#18 /var/www/html/public/index.php(11): require('/var/www/html/v...')
#19 {main}

Description

The error occurs when using both craft.app.session and the cookies plugin in templates that fetch data from an external database. The error is intermittent and can be resolved by reloading the page or clearing the cache with ctrl+F5. However, the error persists when devMode is disabled.

Possible Causes

Based on the provided information, the following possible causes can be identified:

Session handling: The error occurs when using both craft.app.session and the cookies plugin. This suggests that there may be an issue with session handling in Craft 5.
External database connection: The error occurs when fetching data from an external database. This suggests that there may be an issue with the external database connection or the data being fetched.
Cache issues: The error can be resolved by reloading the page or clearing the cache with ctrl+F5. This suggests that there may be cache issues in Craft 5.

Troubleshooting Steps

To troubleshoot this issue, the following steps can be taken:

Check session handling: Verify that session handling is properly configured in Craft 5. Check the session settings in the Craft 5 configuration file.
Check external database connection: Verify that the external database connection is properly configured in Craft 5. Check the database connection settings in the Craft 5 configuration file.
Check cache settings: Verify that cache settings are properly configured in Craft 5. Check the cache settings in the Craft 5 configuration file.
Enable devMode: Enable devMode to see if the error persists.
Clear cache: Clear the cache using ctrl+F5 to see if the error resolves.

Conclusion

The error session_start(): Unexpected end of serialized data in Craft 5 is likely caused by issues with session handling, external database connection, or cache settings. By following the troubleshooting steps outlined above, the issue can be identified and resolved.

Additional Information

Craft CMS version: 5.6.11
PHP version: 8.2
Operating system and version: Not provided
Database type and version: Not provided
Image driver and version: Not provided
Installed plugins and versions: Not provided

Next Steps

If the issue persists after following the troubleshooting steps, further investigation may be required. This may involve:

Checking Craft 5 documentation: Verify that the issue is not a known bug in Craft 5.
Checking plugin documentation: Verify that the issue is not a known bug in the cookies plugin.
Seeking community support: Reach out to the Craft 5 community for further assistance.
Contacting Craft support: Reach out to Craft support for further assistance.
[5.x]: session_start(): Unexpected end of serialized data

Q&A

Q: What is the error `session_start(): Unexpected end of serialized data` in Craft 5?

A: The error session_start(): Unexpected end of serialized data in Craft 5 occurs when there is an issue with session handling, external database connection, or cache settings.

Q: What are the possible causes of this error?

A: The possible causes of this error are:

Session handling: The error occurs when using both craft.app.session and the cookies plugin.
External database connection: The error occurs when fetching data from an external database.
Cache issues: The error can be resolved by reloading the page or clearing the cache with ctrl+F5.

Q: How can I troubleshoot this issue?

A: To troubleshoot this issue, follow these steps:

Check session handling: Verify that session handling is properly configured in Craft 5.
Check external database connection: Verify that the external database connection is properly configured in Craft 5.
Check cache settings: Verify that cache settings are properly configured in Craft 5.
Enable devMode: Enable devMode to see if the error persists.
Clear cache: Clear the cache using ctrl+F5 to see if the error resolves.

Q: What are the next steps if the issue persists?

A: If the issue persists after following the troubleshooting steps, further investigation may be required. This may involve:

Checking Craft 5 documentation: Verify that the issue is not a known bug in Craft 5.
Checking plugin documentation: Verify that the issue is not a known bug in the cookies plugin.
Seeking community support: Reach out to the Craft 5 community for further assistance.
Contacting Craft support: Reach out to Craft support for further assistance.

Q: What are the system requirements for Craft 5?

A: The system requirements for Craft 5 are:

Craft CMS version: 5.6.11
PHP version: 8.2
Operating system and version: Not provided
Database type and version: Not provided
Image driver and version: Not provided
Installed plugins and versions: Not provided

Q: How can I prevent this error from occurring in the future?

A: To prevent this error from occurring in the future, follow these best practices:

Regularly update Craft 5: Regularly update Craft 5 to ensure you have the latest bug fixes and security patches.
Verify session handling: Verify that session handling is properly configured in Craft 5.
Verify external database connection: Verify that the external database connection is properly configured in Craft 5.
Verify cache settings: Verify that cache settings are properly configured in Craft 5.

Q: What are the benefits of using Craft 5?

A: The benefits of using Craft 5 include:

Improved performance: Craft 5 is designed to be faster and more efficient than previous versions.
Enhanced security: Craft 5 includes improved security features to protect your site and its users.
Better support: Craft 5 includes improved support for plugins and themes, making it easier to extend and customize your site.

Q: How can I get help with Craft 5?

A: If you need help with Craft 5, you can:

Check the Craft 5 documentation: The Craft 5 documentation includes a comprehensive guide to getting started with Craft 5.
Join the Craft 5 community: The Craft 5 community is a great resource for getting help and advice from other Craft 5 users.
Contact Craft support: If you need further assistance, you can contact Craft support for help.