need help on auto reply message for contact form

I have my contact form which works out so nicely. and now i just want to add auto reply text to those email id which send me message through contact form. can you guys please help me to add auto reply message on this contact form code??
my contact form php code is as follow

PHP Code:

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<?php 
$myemail ='admin@masen.tk';// <<=== update to your email address
$gmail ='masenlorzen@gmail.com';
session_start();
$errors = '';
$name = '';
$visitor_email = '';
$user_message = '';

if(isset($_POST['submit']))
{
    
    $name = $_POST['name'];
    $visitor_email = $_POST['email'];
    $user_message = $_POST['message'];
    ///------------Do Validations-------------
    if(empty($name)||empty($visitor_email))
    {
        $errors .= "\n Name and Email are required fields. ";    
    }
    if(IsInjected($visitor_email))
    {
        $errors .= "\n Bad email value!";
    }
    if(empty($_SESSION['6_letters_code'] ) ||
      strcasecmp($_SESSION['6_letters_code'], $_POST['6_letters_code']) != 0)
    {
    //Note: the captcha code is compared case insensitively.
    //if you want case sensitive match, update the check above to
    // strcmp()
        $errors .= "\n The captcha code does not match!";
    }
    
    if(empty($errors))
    {
        //send the email
        $to = $gmail;
        $subject="New form submission";
        $from = $myemail;
        $ip = isset($_SERVER['REMOTE_ADDR']) ? $_SERVER['REMOTE_ADDR'] : '';
        
        $body = "A user  $name submitted the contact form:\n".
        "Name: $name\n".
        "Email: $visitor_email \n".
        "Message: \n ".
        "$user_message\n".
        "IP: $ip\n";    
        
        $headers = "From: $from \r\n";
        $headers .= "Reply-To: $visitor_email \r\n";
        
        mail($to, $subject, $body,$headers);
        
        header('Location:thank-you.html');
    }
}

// Function to validate against any email injection attempts
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;
  }
}
?>

<?php 
$title = "Contact Us";
$description = "if you have any suggestion/question then contact me.If you found any mislink on any webpage then please don't hegitate to contact me";
$keywords ="contact, contact me, feedback,masenz.tk,masen.tk,masenz,masen,suggestion, suggest me, support me, support,masen.tk,masenz.tk";

if(!empty($errors)){
echo "<p class='err'>".nl2br($errors)."</p>";
}
?>
<script language="JavaScript" src="http://www.masen.tk/trial/form/1/scripts/gen_validatorv31.js" type="text/javascript"></script>
<div id='contact_form_errorloc' class='err'></div>
<form method="POST" name="contact_form" 
action="html-contact-form.php"> 
<p>
<label for='name'>Name: </label><br>
<input type="text" name="name" value='<?php echo htmlentities($name) ?>'>
</p>
<p>
<label for='email'>Email: </label><br>
<input type="text" name="email" value='<?php echo htmlentities($visitor_email) ?>'>
</p>
<p>
<label for='message'>Message:</label> <br>
<textarea name="message" rows=8 cols=30><?php echo htmlentities($user_message) ?></textarea>
</p>
<p>
<img src="captcha_code_file.php?rand=<?php echo rand(); ?>" id='captchaimg' ><br>
<label for='message'>Enter the code above here :</label><br>
<input id="6_letters_code" name="6_letters_code" type="text"><br>
<small>Can't read the image? click <a href='javascript: refreshCaptcha();'>here</a> to refresh</small>
</p>
<input type="submit" value="Submit" name='submit'>
</form>
<script language="JavaScript">
// Code for validating the form
// for details
var frmvalidator  = new Validator("contact_form");
//remove the following two lines if you like error message box popups
frmvalidator.EnableOnPageErrorDisplaySingleBox();
frmvalidator.EnableMsgsTogether();

frmvalidator.addValidation("name","req","Please provide your name"); 
frmvalidator.addValidation("email","req","Please provide your email"); 
frmvalidator.addValidation("email","email","Please enter a valid email address"); 
</script>
<script language='JavaScript' type='text/javascript'>
function refreshCaptcha()
{
    var img = document.images['captchaimg'];
    img.src = img.src.substring(0,img.src.lastIndexOf("?"))+"?rand="+Math.random()*1000;
}
</script>
</body>
</html>

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