magento-collection.php

<?php
public function create()
{
    $productCollection = $this->_getProductCollection();
    $pages = $productCollection->getLastPageNumber();
    $pageNumber = 1;
    do {
        $productCollection->setCurPage($pageNumber);
        $productCollection->load();
        foreach ($productCollection as $product) {
            // Process product information and write it to file....
        }
        $pageNumber++;
        $productCollection->clear();
        $productCollection = $this->_getProductCollection();

    } while ($pageNumber <= $pages);
}