AMP contact form validation and submission

contact_process.php

<?php
/*
 Function siteURL() courtesy of Chris McKee - https://gist.github.com/ChrisMcKee/1284052
 Get sute URL with protocol (http or https)
*/
function siteURL()
{
    $protocol = (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off'
     || $_SERVER['SERVER_PORT'] == 443) ? "https://" : "http://";
    $domainName = $_SERVER['HTTP_HOST'];
    return $protocol.$domainName;
}

// -> Example.com
function domainName()
{
    return ucfirst($_SERVER['SERVER_NAME']);
}
// cleaning the string
 function clean_string($string) {
      $bad = array("content-type","bcc:","to:","cc:","href");
      return str_replace($bad,"",$string);
    }
// End of functions 


$domain_name = domainName();  // Example.com
$siteURL = siteURL(); //https://example.com
define('RECEIVER_EMAILS', 'example@example.com');
    // Message
$email_subject = "You have a new message from ".clean_string($domain_name);
      

    $name = $_POST['name']; // required
    $message = $_POST['message']; // required
    $email = $_POST['email']; // required
    $phone = $_POST['phone']; // not required
 
    $email_message = "Form details below.\n\n";
 
    $email_message .= "Message: ".clean_string($message)."\n";
    $email_message .= "Name: ".clean_string($name)."\n";
    $email_message .= "Email: ".clean_string($email)."\n";
    $email_message .= "Phone: ".clean_string($phone)."\n";
 
// create email headers
$headers = 
'Reply-To: '.$email."\r\n";



  /* 
  AMP boilerplate CORS code and setting the content type as JSON
     Since all errors / sucess messages will be encoded in JSON
 */
  header("Access-Control-Allow-Origin: ". 
    str_replace('.', '-',''.$siteURL.'') .".cdn.ampproject.org");
  



 header("AMP-Access-Control-Allow-Source-Origin:$siteURL");


 header("Access-Control-Expose-Headers: AMP-Access-Control-Allow-Source-Origin");
 header("Access-Control-Allow-Credentials: true");
 header("Content-Type:  application/json");

 /*
    Check whether message, phone, name and email is filled in
    Do not send the email if not. And send a JSON string to 
    display the errors on the front end. 
 */



    if(!isset($_POST['message']) || empty($_POST['message']) ||
        !isset($_POST['phone']) ||  empty($_POST['phone']) ||
        !isset($_POST['name']) || empty($_POST['name']) ||
        !isset($_POST['email']) || empty($_POST['email']) ) {
       
        $error_array = ['verifyErrors'=>[['name' => 'empty fields','message' => 'Populate All The Fields']]]; // error to be displayed in amp - mustache
        header("Content-Type:  application/json");
        echo json_encode($error_array); 
        header("HTTP/1.1 400 Bad Request");
    }else{
// If there are no errors, submit the form. 
if(mail(RECEIVER_EMAILS, $email_subject, $email_message, $headers))
{
  $data = ['submit-sucess'=>'Success!'];
  echo json_encode($data);
}
}