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 4.0 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 5 running on Windows. Since version 4.0 the SQLSRV extension is compatilbe only with PHP 7.0 running on Linux or Windows.