I would like a back button on the product info page which would go back to the previous category page.

For example :

category 1 > page 5 > product info > back button > back to page 5 of category 1

I stumbled on this back button but it just takes you back to the index page.
PHP Code:
<div class="buttonRow"><?php echo zen_back_link() . zen_image_button(BUTTON_IMAGE_BACK, BUTTON_BACK_ALT) . '</a>'; ?></div>
Any ideas?