Actually, you were very close. The error indicates that Apache is serving PHP files as plain text, which means PHP is not connected to Apache properly. The next issue with mysqli means PHP is running but the MySQL extension is not loaded.
Check:
- Apache PHP module/FastCGI configuration
- The active
php.ini file (php --ini)
- Whether
extension=mysqli is enabled in the PHP configuration used by Apache
- Restart Apache after changes
