position.php

<?php

// Set product positions within a selected category

$cat_id = CATEGORY_ID;

$category = Mage::getModel('catalog/category')->load($cat_id);    

$cat_api = new Mage_Catalog_Model_Category_Api;

$products = $category->getProductCollection();

foreach ($products as $product){

  $new_order = NEW_POSITION;
  
  $cat_api->assignProduct($category->getId(), $product->getId(), $new_order);

}