PDO doesn't close the connection

gistfile1.txt

<?php
$user = 'docker';
$pass = 'mysql';
$dsn = "mysql:host=mariadb;dbname=mysql";
$select = 'SELECT  * from time_zone';
$conn=new PDO($dsn,$user,$pass);
$data = $conn->query($select);
foreach($data as $row) {
        print_r($row);
}
$conn=null;
gc_collect_cycles();
sleep(100)