Using mssql serial command in PHP 5.3

I am using the EasyPHP5.3.0 and MS SQL Server Management Studio Express 
9.00.2047.00

Config of EasyPHP5.3.0
==================
PHP 5.3.0
Apache 2.2.13
MySQL 5.1.37
PhpMyAdmin 3.2.1

I want to use mssql command (e.g. mssql_connect) as my db connector. When I 
run my code the following error has prompted. Can someone tells me what I 
can do?

Fatal error: Call to undefined function mssql_connect() in 
C:\EasyPHP5.3.0\www\ATIS\dbfun.php on line 70

Thank you.

Best Regards

Beo Tse