I'm putting up a new instance of wordpress (WP) on a system running XAMPP. I defined a new database (DB_NAME) and user (DB_USER) as instructed in installation instructions. wp-config.php was created as instructed with the previously defined values for DB_NAME and DB_USER. When trying to run the wp-admin/install.php an error is produced saying the db connection failed. Message suggests verifying that correct values for DB-NAME and DB_USER have been specified. I can verify that the correct values work with MySQL by using MySQL Workbench to connect using the specified values. Then I change wp-config.php to specify DB_USER=root (i.e., the root user for the MySQL server) instead of the name I setup for the wordpress DB and install runs just fine.
Based on the above I conclude WP is unable to handle the value that I chose for DB_USER. I did opt to specify a value that used mixed case (some upper and some lower). This seems to be fine with MySQL but possibly not with WP.