php pdo: getting more detail from an update statement - php

Question

I've been trying to track down a bug in my software for months now, to no success. It all comes down to one update query in PHP PDO seeming to run, with the correct data and yet it doesn't seem to update as "rowcount" returns 0.
Seems entirely random, 99% of the time it runs perfectly. 1% just doesn't update and I've literally no idea why.
For reference, here's the code in case someone spots me doing something really dumb. The part that is having issues is where it's updating the "saved_sessions" table.
// if they have a stay logged in cookie log them in
function stay_logged_in()
{
if (isset($_COOKIE['session']))
{
$session_check = $this->db->run("SELECT `session_id`, `device-id`, `user_id`, `expires` FROM `saved_sessions` WHERE `session_id` = ? AND `expires` > NOW()", array($_COOKIE['session']))->fetch();
if ($session_check)
{
// login then
$this->user_details = $this->db->run("SELECT ".$this::$user_sql_fields." FROM `users` WHERE `user_id` = ?", array($session_check['user_id']))->fetch();
$this->check_banned();
// update IP address and last login
$this->db->run("UPDATE `users` SET `ip` = ?, `last_login` = ? WHERE `user_id` = ?", array(core::$ip, core::$date, $this->user_details['user_id']));
// update their stay logged in cookie with new details
$generated_session = md5(time() . mt_rand() . $this->user_details['user_id'] . $_SERVER['HTTP_USER_AGENT']);
$expires_date = new DateTime('now');
$expires_date->add(new DateInterval('P30D'));
$update_session_sql = "UPDATE
`saved_sessions`
SET
`session_id` = ?,
`expires` = ?
WHERE
`session_id` = ? AND `user_id` = ?";
$update_session_db = $this->db->run($update_session_sql, array($generated_session, $expires_date->format('Y-m-d H:i:s'), $_COOKIE['session'], $session_check['user_id']));
$check_update = $update_session_db->rowcount();
// database was updated, so we can update the cookie
if($check_update == 1)
{
$cookie_domain = false; // allows cookies for localhost dev env
$secure = 0; // allows cookies for localhost dev env
if (!empty($this->core->config('cookie_domain')))
{
$cookie_domain = $this->core->config('cookie_domain');
$secure = 1;
}
setcookie('session', $generated_session, time()+$this->cookie_length, '/', $cookie_domain, $secure);
}
else
{
// logging for me to attempt to check the details match up (which they always seem to do!)
error_log("Couldn't update saved session for user_id " . $session_check['user_id'] . "\n" . "Current user session data: \n" . print_r($session_check, true) . "\nUser cookie data: " . $_COOKIE['session'] . "\n Database info: " . print_r($update_session_db, true));
}
$this->register_session($generated_session, $session_check['device-id']);
return true;
}
else
{
setcookie('session', "", time()-60, '/');
setcookie('device', "", time()-60, '/');
return false;
}
}
else
{
return false;
}
}