What you'll want to do is use a filename that's generated uniquely for each user. There are lots of ways for generating a unique key and we don't need to go into that here.

So generate the key, save the data in a filename that's constructed from the key and pass the key to the user. Use a cookie, or a hidden form element, or something.

Later, when you need to get the data back, use the key that's hidden in the form or from the cookie.