how to save and export data from html to xml file?

I need to make a signup form in HTML. The form must contain 2 textboxes: 1 for the name and 1 for the birthday. And another 2 buttons: 1 for the submit and 1 for the export to xml. I have searched a lot of examples, but I guess none of them can help me.. In this activity, we can only use HTML, CSS, JavaScript, PHP. Is there any possible way to save the form data to an xml file when the user clicks on the submit button? and the xml file should be ready for download when the user clicks on the export to xml?

I really need your help. Thank you very much in advance!