$serverName = "serverName";
$dbusername = "dbusername";
$dbpassword = "dbpassword";
$dbname = 'dbname';

$connectionInfo = array( "Database"=>$dbname, "UID"=>$dbusername, "PWD"=>$dbpassword);

$link = sqlsrv_connect( $serverName, $connectionInfo);

if( $link !== false ) {
  echo "Connection established.".PHP_EOL;}
else{
  die( print_r( sqlsrv_errors(), true));
}

Array(
    [0] => Array
        (
            [0] => IMSSP
            [SQLSTATE] => IMSSP
            [1] => -49
            [code] => -49
            [2] => This extension requires the Microsoft ODBC Driver 11 or 13 for SQL Server. Access the following URL to download the ODBC Driver 11 or 13 for SQL Server for x64: http://go.microsoft.com/fwlink/?LinkId=163712
            [message] => This extension requires the Microsoft ODBC Driver 11 or 13 for SQL Server. Access the following URL to download the ODBC Driver 11 or 13 for SQL Server for x64: http://go.microsoft.com/fwlink/?LinkId=163712
        )
    [1] => Array
        (
            [0] => IM002
            [SQLSTATE] => IM002
            [1] => 0
            [code] => 0
            [2] => [Microsoft][ODBC Driver Manager] Data source name not found and no default driver specified
            [message] => [Microsoft][ODBC Driver Manager] Data source name not found and no default driver specified
        )
)