Description:
------------
It turns out that the fourth parameter (I haven't verified for the others) appears to function "on reference"; i.e. it changes the variable types of set flags from booleans into integers. But what's more, it also changes the variable types on unrelated variables as well.

Test script:
---------------
$dsn = "mysql:host=127.0.0.1;dbname=application_base";
$user = "root";
$password = "";
$flags = [
	PDO::MYSQL_ATTR_FOUND_ROWS		=> true,
	PDO::MYSQL_ATTR_LOCAL_INFILE	=> true
];

$std = new StdClass();
$std->flags = $flags;

// The values are still booleans.
var_dump($flags);

new PDO($dsn, $user, $password, $flags);

// Booleans have turned into integers (as if the $flags parameter is on reference, which would be undocumented).
var_dump($flags);
// Booleans have ALSO turned into integers! Which does not make sense, even if the $flags parameter had been on reference.
var_dump($std->flags);


Expected result:
----------------
The array in the last two var dumps only contains booleans, not integers.

Actual result:
--------------
The array in the last two var dumps suddenly converted both integers into booleans and there is no way to work around that.