QueryMonitor Fatal error: Cannot declare class QM_PHP, because the name is already in use

Case: Staging and production copies of a WordPress 5.4 website on the same server. Activating QueryMonitor 3.9.0 on Staging work, but throws `Fatal error: Cannot declare class QM_PHP` on Production.

Solution

Remove the file: /wp-content/db.php

Source

• https://wordpress.org/support/topic/cannot-activate-query-monitor-php-error-cannot-redeclare-qm_php/