Check if you have the right authentication mode set on the MSSQL Server:
https://msdn.microsoft.com/en-us/library/ms188670.aspx
Also you have two ways to connect to MSSQL through PHP/PDO by using the PHP_PDO_ODBC extension which uses the ODBC driver given in the connectionstring or use PHP_PDO_SQLSRV_xx_TS or PHP_PDO_SQLSRV_xx_NTS extension which you can find here (only for 32bit PHP!) https://www.microsoft.com/en-us/download/details.aspx?id=20098 or use the unofficial 64bit here http://robsphp.blogspot.nl/2012/06/unofficial-microsoft-sql-server-driver.html
connection string when using PHP_PDO_SQLSRV_xx_(N)TS extension:
$hostname='127.0.0.1';
$dbname='test';
$username='user';
$password='pw';
$dbDB = new PDO("sqlsrv:Server=$hostname;Database=$dbname", $username, $password);
connection string when using PHP_PDO_ODBC extension:
//use any of these or check exact MSSQL ODBC drivername in "ODBC Data Source Administrator"
$mssqldriver = '{SQL Server}';
$mssqldriver = '{SQL Server Native Client 11.0}';
$mssqldriver = '{ODBC Driver 11 for SQL Server}';
$hostname='127.0.0.1';
$dbname='test';
$username='user';
$password='pw';
$dbDB = new PDO("odbc:Driver=$mssqldriver;Server=$hostname;Database=$dbname", $username, $password);
When testing a simple query which returned 66 records using PHP_PDO_ODBC extension took ~500ms (for all three MSSQL ODBC drivers) but when using the 64bit(!) PHP_PDO_SQLSRV_TS it took ~5000ms. 10 times slower! Have not yet tried 32bit or the NTS variant.
My dev PC is Windows 7 SP1 using WAMPx64 PHP 5.5.12 and I used PHP_PDO_SQLSRV_55_TS