Instead of pointing to the relative path for productdetails.php (without the /) have you tried pointing to the absolute path /productdetails.php for each product listed? that would force it to look off of the root directory of the site and not the relative directory (in this case product, which doesn't exist). I usually setup a var called 'domain' which contains the domain of the site I am working in, in this case it would be set to = and put this variable before all my links. This makes the code portal since you just need to change this one var when you move to a different URL or allows you to set this to a deeper directory in case you are using a dev server or somthing like domain =
this can really help with issues like this because then you will know you are always pointing to the full domain path.