Codice PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <title>Form</title>
        <style type="text/css"></style>
    
        <script type="text/javascript">
        function cambiaAction(){
        document.getElementById('form').action='index.php';
        alert(document.getElementById('form').action); /*verifica debug*/
        }
        </script>
    </head>
    <body>
        <form id="form" action="">
        <fieldset>
            <input type="button" onclick="cambiaAction()" />
            <input type="submit" />
        </fieldset>
        </form>
    </body>
</html>