According to Microsoft, this extension works on Linux:
https://github.com/Microsoft/msphpsql
(the introduction above does not mention this, rather it says "when running PHP on Windows")
The SQLSRV extension allows you to access Microsoft SQL Server and SQL Azure databases when running PHP on Windows. The 3.0 release of the driver supports SQL Server, beginning with SQL Server 2005, including SQL Server 2012 and SQL Server 2012 LocalDB. (For more information about LocalDB, see » PHP Driver for SQL Server Support for LocalDB and » SQL Server 2012 Express LocalDB.)
The SQLSRV extension is supported by Microsoft and available for download here: » http://msdn.microsoft.com/en-us/sqlserver/ff657782.aspx. SQL Server 2012 LocalDB can be downloaded here: » http://go.microsoft.com/fwlink/?LinkID=237665.
According to Microsoft, this extension works on Linux:
https://github.com/Microsoft/msphpsql
(the introduction above does not mention this, rather it says "when running PHP on Windows")
Microsoft has now committed a PHP 7 pre-release beta of the SQLSRV driver on github:
https://github.com/Azure/msphpsql/tree/PHP-7.0
It looks like the PDO driver is their next target. Anyone interested in using mssql and php7 might want to follow/star that repository.