Comment on PageRank Sculpting by SEO Dave.
Google treats a 301 redirect which many redirect scripts for linking to affiliate products use. Google treats the 301 redirect just like a standard text link with the 0.85 dampening factor like you see with PageRank formula. So nothing to be gained PageRank wise using 301 redirects to link to webpages you don’t want to pass link benefit to.
Because you are linking to a page and that page 301 redirects you are in effect adding the dampening factor twice.
What you are doing is
Linking to Page 1 (0.85 PR cost) which 301 redirects to Page 2 (0.85 PR cost).
Most PHP redirect scripts are not the answer to PR sculpting, the Stallion SEO package includes a redirect feature that cuts out the linking to Page 1 part, so no PR is passed.
Sometimes PHP scripts use 302 redirects (temporary redirect) which used to cause SERPs hi-jacking problems!