Custom Sharing - Re-Directs To Landing Page After Sharing

We are a record label and require a custom sharing page offering fans an incentive for sharing a link to download one of our mp3s We'd like fans to see one webpage and after sharing the current URL link via a Twitter or Facebook share, they are redirected to another webpage. The 2nd webpage must be hidden UNTIL a share is completed. For fans that share the URL of a free mp3 link, we give them another free download. This should be achieved using: PHP / Javascript / API - Must be secure - Work on all browswers - Twitter & Facebook Api Small project and low budget. Thankyou.