Description:
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Problem happens if I request big table or table with lots of data in rows.
Memory usage increasing drastically while fetching one row at a time. 
If I query only one field, ex id, memory usage grows slower.
The code in example gives "PHP Fatal error:  Allowed memory size" after fetching ~175 rows.
Test example generates table, fill it and tries to fetch data row by row.
Tested the code in php 7.2 and it worked as expected.

Test script:
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<?php
$db_name = '';
$db_user = '';
$db_pass = '';
ini_set('memory_limit', '10M');
$cn = 0;
$pdo = new \PDO('mysql:host=localhost;dbname=' . $db_name, $db_user, $db_pass);
$pdo->setAttribute(\PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, false);
$pdo->exec('DROP TABLE IF EXISTS test_mem_leak');
$pdo->exec('CREATE TABLE IF NOT EXISTS test_mem_leak (id INT AUTO_INCREMENT, txt TEXT, PRIMARY KEY(id))');
$fill = str_repeat('test-', 10000);
for ($i = 0; $i < 500; ++$i) {
    $pdo->exec("INSERT INTO test_mem_leak SET txt = '$fill'");
}
$q = $pdo->query('select * from test_mem_leak');
while ($d = $q->fetch()) {
    echo $cn . ' ' . round(memory_get_usage() / (1024 * 1024), 3) . "MB \n";
    ++$cn;
}
echo "ok: $cn\n";


Actual result:
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PHP Fatal error:  Allowed memory size of 10485760 bytes exhausted (tried to allocate 53248 bytes) in /home/me/code/tmp/1.php on line 16