Si hubieras leído bien los requerimientos sabrías por que tienes ese problema.
Cita: For versions 3.0 and 2.0, the x86 version of SQL Server Native Client is required on the computer where PHP is running. If you are using a 64-bit operating system, the x86 version of SQL Server Native Client will be installed with the x64 version of SQL Server Native Client (do not install the x86 version of SQL Server Native Client on an x64 operating system).
If you are using the SQLSRV driver, sqlsrv_client_info will return information about which version of SQL Server Native Client is being used by the Microsoft Drivers for PHP for SQL Server. If you are using the PDO_SQLSRV driver, you can use PDO::getAttribute to discover the version.
Version 3.0 of the driver requires Microsoft SQL Server 2012 Native Client. You can download Microsoft SQL Server 2012 Native Client from the SQL Server 2012 feature pack page.
Version 2.0 of the driver requires Microsoft SQL Server 2008 R2 Native Client. Click on the appropriate link below: