 Re: PHPShop
SKU is implemented in different ways on different e-commerce stores and is largely dependent on how the store owner wants to use SKUs. It also depends on how the application's queries are created for searching for products and how the result lists for the searches for products are displayed. Most applications use product_id internally to find specific products in a database, but some use SKU instead of product_id (e.g., osCommerce's or Zencart's use of the Model field). I'm not sure how phpShop handles SKUs but it's probably along similar lines.
For example, say your store sells multiple product lines and you use a combination of alpahbetic letters and and numbers to create the SKU. For example, you might use base numbers like CP-100015 for computer goods or EQ-100201 for entertainment equipment. Some stores expand on this concept be adding additional information such as product colors. For example, a particular shirt might have a base SKU of CL-1000015 and there may be variants of that SKU such as CL-1000015-White, CL-1000015-Blue, CL-1000015-Beige, etc.
On more complex sites where custom queries are supported, SKUs can be used to reduce search time. For example, it's possible to search for all items using the LIKE operator with an SKU beginning with CP- or all SKUs with the word Blue.
Generally, it is treated as as internal stock numbering system for an e-commerce store, but there are also cases where some applications append the SKU to the end of the auto-numbered product_id of the product's record. I have not seen this done in PHP e-commerce solutions but it is done in commercial applications like StoreFront for ASP and ASP.NET.
A book seller might use the ISBN number of a book as an SKU. A store might use the SKU to store the manufactuer or vendor number of a product. Basically SKUs are intended to give the store owner the freedom to create their own stock numbering system for products and to search for products based on SKU.
A few e-commerce applications allow multiple SKU fields. For example, some may use one SKU to store the shop owners catalog number and might provide additional fields for storing manufacturer or vendor product codes. | 
PHPShop 
Linear Mode
