Take note that Microsoft does not yet (as of 2012-08-15) provide any 64-bit release for their 2.0 or 3.0 drivers. They are 32-bit only.
This will cause problems while loading the the sqlsrv extension with a 64-bit apache installation, like WAMP, as it will simply give the error:
PHP Warning: PHP Startup: Unable to load dynamic library 'c:/wamp/bin/php/php5.4.3/ext/php_sqlsrv_54_ts.dll' - %1 is not a valid Win32 application.\r\n in Unknown on line 0
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 8 driver files, four of which are for PDO support. If you are running non-thread-safe PHP (PHP 5.3), use the php_sqlsrv_53_nts.dll file. (You should use a non-thread-safe version if you are using IIS as your web server). If you are running thread-safe PHP, use the php_sqlsrv_53_ts.dll file. Similarly for PHP 5.4, use the php_sqlsrv_54_nts.dll or php_sqlsrv_54_ts.dll depending on whether your PHP installation is non-thread-safe or thread-safe.
The most recent version of the driver is available for download here: » SQLSRV 3.0 download. If you need support for PHP 5.2 and/or PHP compiled with VC6, use the 2.0 release of the driver: » SQLSRV 2.0 download.
For more information about SQLSRV requirements, see » SQLSRV System Requirements.
The SQLSRV extension is only compatible with PHP running on Windows.