When connection to mssql server with pdo_sqlsrv driver you get:

 
[PDOException]
SQLSTATE[IMSSP]: The DSN string ended unexpectedly.

Sample $connectionOptions:

$connectionOptions = array(
    'dbname' => 'AdventureWorks',
    'user' => 'sa',
    'password' => 'sa_password',
    'host' => 'local',
    'driver' => 'pdo_sqlsrv'
);

Error is in Doctrine\DBAL\Driver\PDOSqlsrv\Driver in function _constructPdoDsn(array $params).
On line 53 dsn starts with

$dsn = 'sqlsrv:(';

instead of

$dsn = 'sqlsrv:server=(';

as written in offical Microsoft help which come with SQL Server Driver For PHP v2.