What version of SQL Server are you running? The ODBC Driver 11 for SQL Server provides support for connecting to Microsoft SQL Server 2005, 2008, 2008R2, 2012, and 2014. I suggest that you check that the SQL Server Native Client is installed on the machine running PHP as well. This is a requirement for using the SQLSRV extension in PHP.
You'll need to download Version 4.0 of the Microsoft Drivers for PHP for SQL Server. Version 4.0 supports PHP 7.0+.