Description:
------------
Fractional seconds are supported in MySQL DateTime(x) field types.
Writing such fields with fractional seconds via PDO into MySQL database is fully supported, however reading such fields directly via PDO won't return fractional seconds. A clumsy workaround is to wrap every DateTime field with a CONCAT function as shown in below test.


Test script:
---------------
(1) Create this table in MySQL:
CREATE TABLE `test` (
  `id` int(11) NOT NULL,
  `dt` datetime(3) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

(2) Connect db via PDO and do this:
$pdo->query("REPLACE INTO test (id, dt) VALUES (1, '2017-1-1 12:30:00.123')");
$a = $pdo->query("SELECT dt FROM test WHERE id = '1'")->fetch();
$b = $pdo->query("SELECT CONCAT(dt) AS dt FROM test WHERE id = '1'")->fetch();
var_dump($a, $b);

Expected result:
----------------
array(1) {
  'dt' =>
  string(23) "2017-01-01 12:30:00.123"
}
array(1) {
  'dt' =>
  string(23) "2017-01-01 12:30:00.123"
}

Actual result:
--------------
array(1) {
  'dt' =>
  string(19) "2017-01-01 12:30:00"
}
array(1) {
  'dt' =>
  string(23) "2017-01-01 12:30:00.123"
}