PDO Memory Leak

test1.php

<?php
// Related SO disscussion:
// https://stackoverflow.com/questions/52348483/pdo-mysqli-memory-leak-on-multiple-select-queries

$dbParams = [
    'host'      => 'localhost',
    'dbname'    => 'playground',
    'username'  => 'homestead',
    'password'  => 'secret',
];

$dsn = "mysql:host={$dbParams['host']};dbname={$dbParams['dbname']}";
$pdo = new \PDO($dsn, $dbParams['username'], $dbParams['password']);
$pdo->setAttribute( \PDO::ATTR_ERRMODE, \PDO::ERRMODE_EXCEPTION );
$pdo->setAttribute(\PDO::ATTR_DEFAULT_FETCH_MODE, \PDO::FETCH_ASSOC);

$pdo->exec('
    CREATE TABLE IF NOT EXISTS temp_table (
      id INT AUTO_INCREMENT PRIMARY KEY,
      text VARCHAR (255) NOT NULL
    )
');

$i = 0;
while (true) {
    $randStr = str_random(255);
    
    $pdo->exec("insert into temp_table(text) values ('$randStr')");
    $id = $pdo->lastInsertId();

    file_put_contents(__FILE__ . '.log', 'Memory: ' . memory_get_usage_in_mb() . PHP_EOL, FILE_APPEND);
    // In test1.php.log:
    // Memory: 0.39
    // Memory: 0.39
    // Memory: 0.39
    // And so 20,000 lines...
    // Without any memory leaks!

    if ($i === 20000) {
        break;
    }
    $i++;
}

function str_random($length = 16)
{
    $string = '';

    while (($len = strlen($string)) < $length) {
        $size = $length - $len;

        $bytes = random_bytes($size);

        $string .= substr(str_replace(['/', '+', '='], '', base64_encode($bytes)), 0, $size);
    }

    return $string;
}

function memory_get_usage_in_mb()
{
    return round(memory_get_usage() / 1024 / 1024, 2);
}

test2.php

<?php

$dbParams = [
    'host'      => 'localhost',
    'dbname'    => 'playground',
    'username'  => 'homestead',
    'password'  => 'secret',
];

$dsn = "mysql:host={$dbParams['host']};dbname={$dbParams['dbname']}";
$pdo = new \PDO($dsn, $dbParams['username'], $dbParams['password']);
$pdo->setAttribute( \PDO::ATTR_ERRMODE, \PDO::ERRMODE_EXCEPTION );
$pdo->setAttribute(\PDO::ATTR_DEFAULT_FETCH_MODE, \PDO::FETCH_ASSOC);

$pdo->exec('
    CREATE TABLE IF NOT EXISTS temp_table (
      id INT AUTO_INCREMENT PRIMARY KEY,
      text VARCHAR (255) NOT NULL
    )
');

$i = 0;
while (true) {
    
    $pdo->query("SELECT 1 as m");

    file_put_contents(__FILE__ . '.log', 'Memory: ' . memory_get_usage_in_mb() . PHP_EOL, FILE_APPEND);
    // In test2.php.log ([line]: message):
    // 1:     Memory: 0.39
    // 5000:  Memory: 0.44
    // 10000: Memory: 0.51
    // 20000: Memory: 0.63
    // Memory leak...

    if ($i === 20000) {
        break;
    }
    $i++;
}

function memory_get_usage_in_mb()
{
    return round(memory_get_usage() / 1024 / 1024, 2);
}