how to write javascript code inside php?

I want the form data to be sent to test.php with the redirection. I can help someone. Thank you.

This is the code:

<?php
    if(isset($_POST['btn'])){
        //do some task
    ?>
    <script type="text/javascript">
        var e = document.getElementById('testForm'); e.action='test.php'; e.submit();</script>
    <?php
    }
    ?>
<form name="testForm" id="testForm"  method="POST"  >
    <input type="submit" name="btn" value="submit" autofocus  onclick="return true;"/>
 </form>

I asked for help on several forums including many StackOverflow but no response. I hope someone here can help me…