If its web hosting then probably cPanel, follow this steps:
1. Login into your cPanel account
2. Search for "Select PHP Version"
3. Select DMNCMS recommend version (try 5.4)
4. Search for mssql - mark V (
You must be registered to see links
)
5. Click save
6. Click set as current
If its Xampp:
1. Open XAMPP Folder
2. Open PHP folder (inside xampp folder)
3. search for php.ini and open it
4. search for ;extension=php_mysql.dll
5. Remove the ; from ;extension=php_mysql.dll
6. Refresh xampp totally - turn off xampp apache service and run it again, or go to services (search at windows) and then find Apache and right click and then restart).