Hack with confidence.
Page back-button-discussion.php is vulnerable to at least the following:
HTML Injection: It is possible to inject your own HTML into this page because the input is not encoded prior to be used as output. Determine which input field contributes output here and inject HTML, CSS, and/or Javascripts in order to alter the client-side code of this page.
Javascript Injection: This page uses at least some of the input from the user to generate Javascript code. Usually in these cases the user input is used to create either a Javascript string or JSON object. Attempt to inject input which when incorporated with the page will form a syntactically correct Javascript statement. This will allow the injection to execute in the context of the browser.
Unvalidated Redirects and Forwards: This page refers directly to dynamic URLs. If the user clicks on one of the link, the URL embedded is passed to a page which performs redirection. Try to over-write one of the intended pages beind passed to redirect the user to an arbitrary page. Give the poisoned link you create to a freind and see if they are redirected to a site of your choosing.
Reflected Cross-Site Scripting: This page is vulnerable to reflected cross-site scripting because the input is not encoded prior to be used as output. Determine which input field contributes output here and inject scripts. Try to redirect the user to the capture-data.php page which records cookies and other parameters. Visit the captured-data.php page to view captured data.