Ebay Dispute Case Refund via API

I need a PHP class where I put my ebay keys along with the token and CaseID and it refund amount the case id.
I tried myself but ebay need Digital Signature and this is where I am stuck.

You can see the documentation here: https://developer.ebay.com/Devzone/post-order/post-order_v2_casemanagement-caseid_issue_refund__post.html