Drupal Commerce: discount for line items based on user's values?
I don't think the line item will be exposed to the PHP block, and I doubt there'll be anything in scope to get a reference back to it. 

You might need to create your own Rules action. It's pretty simple, and you can copy most of what you need from commerce_line_item_unit_price_subtract(), which does have access to the line item entity.

hook_rules_action_info() would be a good place to start.