You're saying it's possible to host this script on another server and still have the added benefits of seo for my domain?
A side effect of using JavaScript is that it is that it is invisible to Search Engines. So the reviews would not be indexed by search engines.
If PHP was used, it would be visible to search engines, since PHP renders the reviews as HTML.
To answer your question above, I know I could use JavaScript to talk to PHP on a different server. In fact I just tested my idea to make sure I'm not talking out of my ear here.
It would very likely require some, if not a lot of, custom code though. You might be able to find a javascript interface that handles the rating system front-end. That goes with a PHP back-end that talks to the database and returns results based on a simple product-id methodology. In this case you could run the javascript on your main site, the php on the remote-secondary site and use a little javascript/php hand-rolled module to make the two talk.
It would likely be less work creating a JavaScript review system, that saves the reviews to a custom PHP script.
Creating a javascript interface for the review script you have may be more work depending on how well they separate the logic from the rendering of HTML.
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A non-standard solution you could use it to have a PHP server save your reviews, but then have that PHP server save those reviews to your site as HTML.
Under the reviews tab on your product pages, you have HTML. If this is custom HTML that you added through a form (maybe in the administration panel of your store), then you can have PHP do this automatically for you.
It could post the HTML for reviews through that form, to be saved for the product.
This would be search engine friendly.
It would require that the PHP server know your administration panel password however, or at least the minimum required access to post HTML under the reviews tab.
It is non-standard, and if your PHP server had a vulnerability it would allow someone to log into your online store also. So I'd be very careful in choosing such a solution - but this is probably the only option you have of getting search engine friendly reviews.