This seems straightforward, but I can't get it to work. I have a button, when it's clicked I'd like to execute a php function which is defined in the same file as the button. My code is below, I get the "clicked" alert when the button is clicked, but no alert on response from the function.
//this is in myfile.php
    <?php
        echo '<button type="button" name="save_button" onclick="save()">Save</button>';
    ?>
        <script type="text/javascript">
            function save() 
            {
              alert("clicked");
              $.ajax({
                url: 'myfile.php',
                type: 'post',
                data: { "set_description": ""},
                success: function(response) { alert(response); }
              });
            }
          </script>
        <?php 
         function set_description() {
          return "a string";
        }
        ?>