I have multiple virtual servers each of which is serving its own installation of WordPress. These have all been working properly for weeks to years.
Today I've been switching servers from Apache mod_php to FCGId if they were not already using FCGId. This has mostly gone relatively smoothly so far, but in a couple of cases, when testing the result of the change by visiting the respective WordPress site, I get the error message "Your PHP installation appears to be missing the MySQL extension which is required by WordPress."
I've looked around and can't see any difference between configurations - can you please help me understand what's gone wrong and how to fix it?