PHP redirect to same page after submit

Question

Please I need your help about my basic php practice. Please check below is my code. I have added action of my form by different php file. I want after submit my form. I will see this message Successfully Submitted after showing this thank you message redirect to index.php page again but now this page don't redirect. It's stay contact.php page. Thanks.

    <?php 
    if(isset($_POST['submit'])){

        $sub = "New Message";
        $to = "musar001@gmail.com";
        $email = $_POST['email'];
        $message = "Your Name:" . " " .$_POST['name'] . "\r\n";
        $message .= "Your E-mail:" . " " . $_POST['email'] . "\r\n";
        $message .= "Mobile Number:" . " " . $_POST['mobile'] . "\r\n";
        $message .= "Country Name:" . " " . $_POST['country'] . "\r\n";
        $message .= "I have dog(s):" . " " . $_POST['dog'] . "\r\n";
        $message .= "And Cat(s):" . " " . $_POST['cat'] . "\r\n";
        $headers = "From: $email" . "\r\n";

        $success = mail($to, $sub, $message, $headers);

        if(isset($_POST) && $success) {
            echo "<h2>Successfully Submitted. Thanks.</h2>";
//If I use header function I can't see thank you message. 
 // header('location: index.php');
        }
    }

?>

Few Example of html code.

<form action="contact.php" method="POST" id="contactform" class="wpcf7-form contactform-v2">

   <div class="row">
      <div class="col-xs-6 col-sm-6">

        <p class="contact-form-author">

  <input type="text" required="required" value="" name="name" id="name" placeholder="Your Name *"></p>

    </div>

          <div class="col-xs-6 col-sm-6">

            <p class="contact-form-email">

              <input type="email" required="required" value="" name="email" id="email" placeholder="Your Email *"></p>

          </div>

      </div>
</form>

I didn't find any answer of my question.??