Email Thank You

I am using this code.  Whenever an email is sent another page pops up (thank_you.html) saying thank you.  I don't want that.  I simply want the input field that previously said "Your Email Here" to now say.  "Thank You".    How can I do this?

www.mediascrubber.com/videos.html

Its at the very bottom of the page.  Subscribe.

I assume I would do an echo "Thank You" of some sort.  But where?

<form class="form-horizontal" action="send_mail.php" method="post">
				<div class="input-group">
                <input type="text" name="email_address" class="def-input" placeholder="Your Email Here">
                <span class="input-group-btn">
                <button class="btn-form btn" input type="submit">Subscribe</button>
                </span>
                <!--input type="submit" value="Submit" /-->
				</div>
                </form>

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<?php
/*
This first bit sets the email address that you want the form to be submitted to.
You will need to change this value to a valid email address that you can access.
*/
$webmaster_email = "ds@mediascrubber.com";

/*
This bit sets the URLs of the supporting pages.
If you change the names of any of the pages, you will need to change the values here.
*/
//$feedback_page = "feedback_form.html";
//$error_page = "error_message.html";
$thankyou_page = "thank_you.html";

/*
This next bit loads the form field data into variables.
If you add a form field, you will need to add it here.
*/
$email_address = $_REQUEST['email_address'] ;
//$comments = $_REQUEST['comments'] ;

/*
The following function checks for email injection.
Specifically, it checks for carriage returns - typically used by spammers to inject a CC list.
*/
function isInjected($str) {
	$injections = array('(\n+)',
	'(\r+)',
	'(\t+)',
	'(%0A+)',
	'(%0D+)',
	'(%08+)',
	'(%09+)'
	);
	$inject = join('|', $injections);
	$inject = "/$inject/i";
	if(preg_match($inject,$str)) {
		return true;
	}
	else {
		return false;
	}
}

// If the user tries to access this script directly, redirect them to the feedback form,
if (!isset($_REQUEST['email_address'])) {
header( "Location: $feedback_page" );
}

// If the form fields are empty, redirect to the error page.
//elseif (empty($email_address) || empty($comments)) {
//header( "Location: $error_page" );
//}

// If email injection is detected, redirect to the error page.
elseif ( isInjected($email_address) ) {
header( "Location: $error_page" );
}

// If we passed all previous tests, send the email then redirect to the thank you page.
else {
mail( "$webmaster_email", "Feedback Form Results",
  $comments, "From: $email_address" );
header( "Location: $thankyou_page" );
}
?>

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