Free PDO statement result from memory in loop

I am using the following code in an application based on ZF1:

$select = $db->select()->from('table', array('id', 'int', 'float'))->limit(10000, (($i - 1) * 10000));
$data = $select->query();
while ($row = $data->fetch()) {
     # ...
}

This operation is happening in a foreach loop for some 800 times. I output the memory usage for each pass and can see it increasing by about 5MB per pass. I suppose that is because Zend apparently does not free the result from the query once the pass is complete. A simple unset didn't solve the issue. Using fetchAll also did not improve (or change) the situation.

Is there any way to free the result from a Zend_Db_Statement_PDO thus freeing the memory used by it? Or do you suspect another reason?

Solution

Actually the problem was hidden somewhere else:

Inside the loop some integer results were stored in an array for modification at a later planned stage in the workflow.
While one might expect PHP arrays to be small, that is not the case: Arrays grow big really fast and a PHP array is on average 18 times larger than it is to be 'expected'. Watch out while working with arrays, even if you only store integers in them!

In case the linked article disappears sometime:

In this post I want to investigate the memory usage of PHP arrays (and values in general) using the following script as an example, which creates 100000 unique integer array elements and measures the resulting memory usage:
$startMemory = memory_get_usage();
$array = range(1, 100000);
echo memory_get_usage() - $startMemory, ' bytes';
How much would you expect it to be? Simple, one integer is 8 bytes (on a 64 bit unix machine and using the long type) and you got 100000 integers, so you obviously will need 800000 bytes. That’s something like 0.76 MBs.

Now try and run the above code. This gives me 14649024 bytes. Yes, you heard right, that’s 13.97 MB - eightteen times more than we estimated.