Final answer:
The error indicates a failure to create a MySQL database in PHP because the database with the intended name already exists.
Step-by-step explanation:
The question relates to an error encountered in MySQL while using PHP and XAMPP, specifically while attempting to create a database that already exists. The error message 'fatal error: uncaught mysqli sql exception: can't create database 'mydb'; database exists' indicates that a database with the name 'mydb' cannot be created because there is already a database with this name in the MySQL server you are attempting to use.
To resolve this issue, you can either choose a different name for your new database, or if the existing database is no longer needed, you can first drop it using the SQL statement DROP DATABASE mydb;. Remember that dropping a database will permanently delete it and all its content, so this action should be done with caution.
Alternatively, if you intended to use this existing database rather than create a new one, you should connect to it without attempting creation using the mysqli::select_db method.
To fix this, choose a different database name or carefully drop the existing one if it's no longer needed, or connect to the existing database without trying to create it.