PHP Fatal error: Uncaught Error: Call to undefined function mysqli_connect()

What the message looks like

PHP Fatal error:  Uncaught Error: Call to undefined function mysqli_connect() in /var/www/example.com/db.php:4
Stack trace:
#0 /var/www/example.com/index.php(2): require()

Common causes

• The package with the MySQL extensions (php-mysql) is not installed.
• It was installed for another PHP version than the one that serves the site, for example after an upgrade with several versions side by side.
• The extension is installed but disabled in the conf.d of the FPM configuration.
• PHP-FPM was not restarted after the installation, so the running workers still lack the module.
• In a container, the extension was never built into the image.

Check

1. Does the CLI PHP know mysqli?
   php -m | grep -i mysqlCopy
2. Which PHP-MySQL packages are installed?
   dpkg -l | grep -E '^ii +php[0-9.]*-mysql'Copy
3. Does each installed PHP-FPM binary list mysqli? (this is what the site uses)
   for f in /usr/sbin/php-fpm*; do echo "$f"; $f -m | grep -i mysqli; doneCopy
4. Is it enabled in the FPM configuration?
   ls -l /etc/php/*/fpm/conf.d/ | grep -i mysqlCopy

Fix

• Install the extension for the version in use: apt install php-mysql, or php8.x-mysql with the exact version name of your FPM.
• Enable it if needed: phpenmod mysqli.
• Restart the PHP-FPM service of that version so the workers load the module.
• Check again with the PHP-FPM binary of that version: php-fpm8.x -m | grep mysqli.
• In a Docker image add docker-php-ext-install mysqli to the Dockerfile and rebuild.

FAQ

Why does it work on the command line but not in the browser?

The CLI and PHP-FPM use different configuration and can even be different PHP versions. Test with the PHP-FPM binary, not only with php -m.

Is the same cause behind Class mysqli not found?

Yes. Both messages come from the missing mysqli extension. The same applies to PDO: could not find driver means pdo_mysql is missing, which the same package provides.