Sequential Encoding (not Base64 )

Hello,

I would really appriciate if you code something than can do soething like Base64 encoding, just a bit shorter and with only numbers.

It is oncredibly good for encoding product catalogs urls for affiliate sites with many products. Base64 is too long to be any good for seo, however something like this would be really nice.

Generated link example: http://mydomains.com/prefix/product_321313131

With base64 i guess the 321313131 would be much longer and it will contain letter.

http://wordpress.org/extend/plugins/wp-noexternallinks/