Description:
------------
If 2 PDO connections are opened they seem to be merged.

If the first is using unbuffered and second using buffered the first results in a buffered return.

 If both connections are using unbuffered and the first connection is running the second connection returns:

Cannot execute queries while other unbuffered queries are active.  Consider using PDOStatement::fetchAll().  Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY attribute.


on the first call.


Issue produces on machines running 7.3.17 (OS Amazon Linux release 2 (Karoo)) and 7.1.33 (OS Red Hat Enterprise Linux Server release 7.3 (Maipo)).

Test script:
---------------
Case 1:

$db = new PDO("mysql:host=".$dbhost.";dbname=".$dbname, $dbuser, $dbpass, array(PDO::ATTR_PERSISTENT => true, PDO::MYSQL_ATTR_USE_BUFFERED_QUERY => false));
$db_ub = new PDO("mysql:host=".$dbhost.";dbname=".$dbname, $dbuser, $dbpass, array(PDO::ATTR_PERSISTENT => true, PDO::MYSQL_ATTR_USE_BUFFERED_QUERY => true));
$stmt = $db->prepare('A QUERY THAT WILL EXCEED MEMORY LIMIT IF ALL ROWS RETURNED');
$stmt->execute();
while($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
//should run out of memory
}

Case 2:

$db = new PDO("mysql:host=".$dbhost.";dbname=".$dbname, $dbuser, $dbpass, array(PDO::ATTR_PERSISTENT => true, PDO::MYSQL_ATTR_USE_BUFFERED_QUERY => false));
$db_ub = new PDO("mysql:host=".$dbhost.";dbname=".$dbname, $dbuser, $dbpass, array(PDO::ATTR_PERSISTENT => true, PDO::MYSQL_ATTR_USE_BUFFERED_QUERY => false));
$stmt = $db->prepare('A QUERY THAT WILL EXCEED MEMORY LIMIT IF ALL ROWS RETURNED');
$stmt->execute();
while($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
$stmt = $db_ub->prepare('select now()');
$stmt->execute();
}

Expected result:
----------------
$stmt should only have 1 row at a time and memory usage should be low

Actual result:
--------------
Usually results in PHP Fatal error:  Allowed memory size ... 

ini_set('memory_limit', can be used to get the script to run.