IIS and PHP Returning Incorrect Session Responses

Question

After a recent migration of a web application, there are occasional instances where users receive incorrect responses from the application. This issue did not occur in the previous environment, which utilized older versions of Windows Server, IIS, PHP, and MSSQL, but the codebase remains unchanged.

The setup includes:

• 1 PHP web application
• 2 main customers, each with a distinct set of users
• 2 MSSQL (2022) databases, with the application selecting the appropriate database based on user login

The problem manifests as a user receiving a response that contains data not associated with their database, and this occurs sporadically. After refreshing the page, users see the correct response again. The user has been tested to ensure they do not have access to the other database.

There is a suspicion that the issue may stem from the interaction between IIS and PHP (FastCGI), given its intermittent nature. The application pool is configured with a maximum worker processes value of 1.

Is this a known issue, and what steps should be taken to diagnose or resolve it? The current environment is:

• Windows Server 2022 Standard (64bit)
• IIS 10.0.20348.1
• PHP 8.2.12 NTS x64 (running through FastCGI)

Any insights on where to start troubleshooting would be greatly appreciated.