Uncaught exception RuntimeException with message The mssql extension is not loaded

I am using PHP 5.3.4 as a front end with MSSQL in the back end. I had enabled relevant extension in php.ini

extension=php_mssql.dll

but even then whenever I am accessing MSSQL db from any web page, the pages are terminated with a fatal error as

"Fatal error: Uncaught exception 'RuntimeException' with message 'The "mssql" extension is not loaded' in ..."

Although MSSQL is running fine from MS SQL Server Studio. I am using XAMPP server 2.1 with Windows 11 64 bit OS.