How do I get the SQLSRV extension to work with PHP, since MSSQL is deprecated?

Quoting http://php.net/manual/en/intro.mssql.php:

The MSSQL extension is not available anymore on Windows with PHP 5.3 or later. SQLSRV, an alternative driver for MS SQL is available from Microsoft: » http://msdn.microsoft.com/en-us/sqlserver/ff657782.aspx.

Once you downloaded that, follow the instructions at this page:

• Loading the Microsoft Drivers for PHP for SQL Server

In a nutshell:

Put the driver file in your PHP extension directory.
Modify the php.ini file to include the driver. For example:

extension=php_sqlsrv_53_nts_vc9.dll  

Restart the Web server.

See Also (copied from that page)

• System Requirements (Microsoft Drivers for PHP for SQL Server)
• Getting Started
• Programming Guide
• SQLSRV Driver API Reference (Microsoft Drivers for PHP for SQL Server)

The PHP Manual for the SQLSRV extension is located at http://php.net/manual/en/sqlsrv.installation.php and offers the following for Installation:

The SQLSRV extension is enabled by adding appropriate DLL file to your PHP extension directory and the corresponding entry to the php.ini file. The SQLSRV download comes with several driver files. Which driver file you use will depend on 3 factors: the PHP version you are using, whether you are using thread-safe or non-thread-safe PHP, and whether your PHP installation was compiled with the VC6 or VC9 compiler. For example, if you are running PHP 5.3, you are using non-thread-safe PHP, and your PHP installation was compiled with the VC9 compiler, you should use the php_sqlsrv_53_nts_vc9.dll file. (You should use a non-thread-safe version compiled with the VC9 compiler if you are using IIS as your web server). If you are running PHP 5.2, you are using thread-safe PHP, and your PHP installation was compiled with the VC6 compiler, you should use the php_sqlsrv_52_ts_vc6.dll file.

The drivers can also be used with PDO.

Download Microsoft Drivers for PHP for SQL Server. Extract the files and use one of:

File                             Thread Safe         VC Bulidphp_sqlsrv_53_nts_vc6.dll           No                  VC6php_sqlsrv_53_nts_vc9.dll           No                  VC9php_sqlsrv_53_ts_vc6.dll            Yes                 VC6php_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.