What does the message "Fatal error: Uncaught PDOException: SQLSTATE[HY000] [1049] Unknown database" mean?

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The message "Fatal error: Uncaught PDOException: SQLSTATE[HY000] [1049] Unknown database" means that the application is trying to access a database that does not exist or is spelled incorrectly. The error can be caused by a misconfiguration of the application or a database configuration file, or by a problem with the database server itself. It is often a result of a simple typo or an error in the database connection settings. In order to fix the issue, the user needs to ensure that the database exists and its name is spelled correctly. Additionally, the user needs to check if the connection settings are correct and if the database server is running.