Programmatically add Magento products to categories

Question

I am using Magento 1.4.0.1. I have over 21000 simple products, each entered into a single category. There are hundreds of categories in my site. Some products belong in mult

Answer 1

In PHP code you can put them into the category while you are importing them.

Say you have a product called $product and a category ID called $category_id

You can set the categories which a product belongs to by doing the following

$categories = array($category_id);
$product->setCategoryIds($categories);
$product->save();

If the product already has categories and you'd like to add one more then you can use getCategoryIds() like this:

$categories = $product->getCategoryIds();
$categories[] = $categoryId;
$product->setCategoryIds($categories);
$product->save();

Or, as mentioned by Joshua Peck in the comments, you can use the category_api model to add or remove a product from a category without affecting it's current category assignments:

Mage::getSingleton('catalog/category_api')
  ->assignProduct($category->getId(),$p‌​roduct->getId());

Mage::getSingleton('catalog/category_api')
  ->removeProduct($category->getId(),$p‌​roduct->getId());