Download Microsoft Drivers for PHP for SQL Server. Extract the files and use one of:
File Thread Safe VC Bulid
php_sqlsrv_53_nts_vc6.dll No VC6
php_sqlsrv_53_nts_vc9.dll No VC9
php_sqlsrv_53_ts_vc6.dll Yes VC6
php_sqlsrv_53_ts_vc9.dll Yes VC9
You can see the Thread Safety status in phpinfo().
Add the correct file to your ext directory and the following line to your php.ini:
extension=php_sqlsrv_53_*_vc*.dll
Use the filename of the file you used.
As Gordon already posted this is the new Extension from Microsoft and uses the sqlsrv_* API instead of mssql_*
Update:
On Linux you do not have the requisite drivers and neither the SQLSERV Extension.
Look at Connect to MS SQL Server from PHP on Linux? for a discussion on this.
In short you need to install FreeTDS and YES you need to use mssql_* functions on linux. see update 2
To simplify things in the long run I would recommend creating a wrapper class with requisite functions which use the appropriate API (sqlsrv_* or mssql_*) based on which extension is loaded.
Update 2: You do not need to use mssql_* functions on linux. You can connect to an ms sql server using PDO + ODBC + FreeTDS. On windows, the best performing method to connect is via PDO + ODBC + SQL Native Client since the PDO + SQLSRV driver can be incredibly slow.