There are 2 supported drivers that allow connections to Microsoft SQL Server databases.

This driver, which runs on both Windows and Linux clients, allows connections to Microsoft SQL Server databases versions 2008 and up, and uses the Microsoft Native Client libraries. This driver is recommended for Windows Clients running PHP 5.3 and higher, and Linux Clients running PHP 7.0 and higher, and provides the fastest PHP connection.

More about the Microsoft SQL Server client for Linux here

This allows connections from Unix based clients using the freetds libraries. It can be used with version 5.x of PHP

More about the FreeTDS libraries here

Connecting to SQL server 2003 creates a difficulty There is no support for native client connections, and the alternative method, using php_mssql.dll was removed in PHP 5.3. The only alternative is to use a generic ODBC driver that does not require the Microsoft Native Client libraries.

This is the preferred driver for connecting to a SQL server database from both Windows and Linux clients.

The original driver was contributed by Microsoft and provides connections to SQL server databases greater than SQL Server 2003.

• On Windows clients, It requires PHP version 5.3 or greater, Microsoft native client software and the appropriate PHP driver.
• On Linux clients, PHP version 7.0 and higher is required.

Sequences are supported in native mode in SQL Sever 2012, and emulated in earlier versions.

The default behavior for the PHP mssqlnative driver is to return date/time fields as a PHP dateTime object, that looks like this:

[RequiredDate] => DateTime Object
(
    [date] => 1996-08-01 00:00:00.000000
    [timezone_type] => 3
    [timezone] => America/Denver
)

However, in order to improve portability, the default behaviour of the ADOdb mssqlnative driver is to return the date as a string:

[RequiredDate] => 1996-08-01 00:00:00.000000

In order to return the dateTime as an object, the following action is required:

$db = ADONewConnection('mssqlnative');
$db->setConnectionParameter('ReturnDatesAsStrings',false);
$db->connect('SERVER\SQLEXPRESS','user','password','NORTHWND');

You can set the character set to UTF-8 using:

$db = ADONewConnection('mssqlnative');
$db->setConnectionParameter('characterSet','UTF-8');
$db->connect('SERVER\SQLEXPRESS','user','password','NORTHWND');

To use Windows, instead of SQL Server authentication, pass NULL as the user and password as shown below

$db = ADONewConnection('mssqlnative');
$db->connect('SERVER\SQLEXPRESS',NULL,NULL,'NORTHWND');

This is the preferred driver for connecting to a SQL server database from Linux/Unix clients. This driver provides access to Microsoft SQL Server databases from unix systems via Freetds

This driver was the primary connection method to SQL Server databases up to PHP Version 5.2. The windows dll associated with this driver (php_mssql.dll) was removed from the standard PHP distribution in PHP Version 5.3. There is no community support for this driver.

This driver provides undocumented portability extensions to the Unix mssql driver

This driver appears to provide Unicode enhancements to the mssql driver