PHP - mysqli_fetch_all() allowed memory size bytes exhausted - what is an alternative to mysqli_fetch_all() which is better for memory

Question

I have a PHP script which returns lots of rows from my MySQL DB. Usually no issue occurs but occasionally a large amount of rows are returned which causes memory exhaustion.

The fetch is here:

$fetchall = mysqli_fetch_all($getPrepData->get_result(), MYSQLI_ASSOC);

I am wondering what an alternative is the mysqli_fetch_all which would be better for memory to prevent this issue without increasing memory limit etc

Update:

I have added the following as suggested in the comments

$result = $getPrepData->get_result();
$data = [];
while ($row = $result->fetch_assoc()) {
     $data[] = $row;
}

But I still get a memory error on this line:

 $result = $getPrepData->get_result();