Running PHP 7.2 on IIS 10, I had to use the IIS Web Platform Components wizard to install the Microsoft PHP drivers version 5.2 x64 to get this command to work. Manually installing 4.0 drivers did not work.
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.
Running PHP 7.2 on IIS 10, I had to use the IIS Web Platform Components wizard to install the Microsoft PHP drivers version 5.2 x64 to get this command to work. Manually installing 4.0 drivers did not work.
PECL packages for Linux SQLSRV and PDO_SQLSRV drivers (4.0.7) are available now.
https://github.com/Microsoft/msphpsql/releases
SQLSRV doesn't use mssql_connect - it is it's own extension, therefore to connect you'd use sqlsrv_connect:
http://www.php.net/manual/en/function.sqlsrv-connect.php