How to update form data from MySQL with OnClick

I'd like advice on the cleanest method to achieve the following, using only Javascript & PHP.

I have a web page with form data which is initially blank. When the user selects from a pulldown (constructed with <select> ... <option>) I want to look up data from MySQL and populate the form. I know how to use OnClick to run a Javascript function, I know how to use Javascript to write data back to the form, and I know how to use PHP to extract data from the MySQL database. How do I combine these?

Simplified code:

    <script type="text/javascript">
      function fillData (rSelect) {
        var tempvar = rSelect.form.Data.value;

/* What I'd really like is to stick some php code here to extract the data from mysql and populate
   the variable(s) like tempvar, but I know that isn't how it works. I'm looking for an equivalent. */

        rSelect.form.Field1.value = tempvar;
      }
    </script>
...
          <form method="post" action="newdata_entry" id="my_form">

          <select size="1" name="Data" OnChange="fillData(this)">
            <option>Text1</option>
            <option>Text2</option>
          </select>
          <textarea rows="20" cols="175" name="Field1" id="my_message"></textarea>
...

Can I use JS to fill variables, then reload the web page and have php run the 2nd time through?