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 8 driver files, four of which are for PDO support.
The most recent version of the driver is available for download here: » SQLSRV download.
For more information about SQLSRV requirements, see » SQLSRV System Requirements.
Since version 4.0 the SQLSRV extension is compatible only with PHP 7.0 running on Linux or Windows.