This is either a bug report or a question regarding the PDO::ATTR_PERSISTENT attribute - depending on what the facts are. My initial question is: is the option supported? The reason I ask is because it appears that you can set the option when you create a new PDO connection, and you can read the option back using getAttribute(PDO::ATTR_PERSISTENT) - seeming to imply that it is supported. But only if you are careful with the order of the options in the call to the PDO constructor.

This, for example, will work and set the option:

$conn = new PDO(
    "sqlsrv:server=$serverName;Database=$database",
    $uid,
    $pwd,
    array(
        PDO::ATTR_PERSISTENT => true,
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
    )
);

But changing the order of the options will throw an exception. This call:

$conn = new PDO(
    "sqlsrv:server=$serverName;Database=$database",
    $uid,
    $pwd,
    array(
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_PERSISTENT => true
    )
);

will result in this exception message and a failure to connect:

SQLSTATE[IMSSP]: An unsupported attribute was designated on the PDO object.

This blog entry from Microsoft when these new drivers were first released noted that ODBC connection pooling is used instead of the persistent connection offered by other older, non-microsoft drivers.

So should one ever use ATTR_PERSISTENT? If yes to that question, why does the second code example throw an exception? If no why is it even exposed by the driver - or at the very least why is it not simply ignored (as I mentioned above, the first code example does succeed in the setting the option)?

FWIW the following versions were used for the testing:

PDO::ATTR_CLIENT_VERSION value:
array (
  'DriverDllName' => 'msodbcsql11.dll',
  'DriverODBCVer' => '03.80',
  'DriverVer' => '12.00.4100',
  'ExtensionVer' => '3.2.0.0',
)

Windows 7 Professional, Service Pack 1
PHP 5.6.13
Apache 2.4.16