Why is it not permissible to use root for MySQL session storage?

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Using root for MySQL session storage is not permissible because it is a security risk. The root user has access to all databases and can perform any action on them. Giving root access to a PHP application by allowing it to store sessions in that user's database allows for potential exploits to occur, as a compromised session could give access to the entire system. It is recommended to create a separate, restricted user with limited privileges for the application to use.