How PHP connect to MSSQL server 7.0?

[liunx]

Hi all,
I'm writing a program by PHP (v 4.0.6)(using Personal Web Server on Win98_SE).
I write:
...
// MSSQL 7.0 SERVER PARAMETERS
$hostname = "dng_com";
$username = "madam";
$password = "kimanh";

// Database
$dbname = "z_pvt";
$tablename = "bcn" ;

// CONNECT TO THE MSSQL 7.0 DATABASE
$connection =
mssql_connect($hostname,$username,$password);
...

When I run this program, It notice: "Fatal error: Call to undefined
function: mssql_connect() "
Why like that?
Please help me!
anha75.