When there are multiple PHP scripts running in parallel, each making an UPDATE query to the same record in the same table repeatedly, is it possible for there to be a 'lag time' before the table is updated with each query?

I have basically 5-6 instances of a PHP script running in parallel, having been launched via cron. Each script gets all the records in the items table, and then loops through them and processes them.

However, to avoid processing the same item more than once, I store the id of the last item being processed in a seperate table. So this is how my code works:

function getCurrentItem()
{
  $sql = "SELECT currentItemId from settings";
  $result = $this->db->query($sql);
  return $result->get('currentItemId');
}

function setCurrentItem($id)
{
   $sql = "UPDATE settings SET currentItemId='$id'";
   $this->db->query($sql);
}

$currentItem = $this->getCurrentItem();

$sql = "SELECT * FROM items WHERE status='pending' AND id > $currentItem'";
$result = $this->db->query($sql);
$items = $result->getAll();

foreach ($items as $i)
{
   //Check if $i has been processed by a different instance of the script, and if so, 
   //leave it untouched.
   if ($this->getCurrentItem() > $i->id) 
     continue;

   $this->setCurrentItem($i->id);
   // Process the item here
}

But despite of all the precautions, most items are being processed more than once. Which makes me think that there is some lag time between the update queries being run by the PHP script, and when the database actually updates the record.

Is it true? And if so, what other mechanism should I use to ensure that the PHP scripts always get only the latest currentItemId even when there are multiple scripts running in parrallel? Would using a text file instead of the db help?