Description:
------------
I've just come across an out of memory issue in a larger CLI-command that does lots calculations across multiple tables. The command ran fine on 7.1.x but errors on 7.2.2. I've nailed it down to the PDOStatement class (using Blackfire.io) sometimes creating 26 kb garbage _per call_.

The attached test script should not create any garbage whatsoever, and even the gc_collect_cycles call does not reduce the memory footprint. Everything works when running it under 7.1. Opcache is disabled (for CLI).

Test script:
---------------
<?php

$dsn = 'mysql:dbname=homestead;host=127.0.0.1';
$user = 'homestead';
$password = 'secret';

$pdo = new PDO($dsn, $user, $password);

for($i = 0; $i < 1000000; $i++) {
    echo memory_get_usage() . "\n";
    $statement = $pdo->prepare('SELECT * FROM demo');
    $statement->execute();

    gc_collect_cycles(); // does not help as well
}


Expected result:
----------------
362504
107678504
107678504
107678504
107678504
107678504
107678504
107678504

Actual result:
--------------
574888
100246184
100246312
100246440
100246568
100246696
100246824
100246952