Process subscription form submissions in AMP

subscriber_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.comP
    $email = $_POST['subscriber_email']; // required
  /* 
  AMP boilerplate CORS code and setting the content type as JSON
     Since all errors / sucess messages will be encoded in JSON
 */
  header("Access-Control-Allow-Origin: ". 
    str_replace('.', '-',''.$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");


    // List of our subscribers' emails
$our_subscribers = ['example@example.com', 'example2@example.com'];
 /*
    Check whether Email is filled in.
    Add error to the error array. And send a JSON string to 
    display the errors on the front end. 
 */
    if(!isset($_POST['subscriber_email']) 
        || empty($_POST['subscriber_email']) ) {
       
        $error_array = ['verifyErrors'
    =>[['name' => 'missing email','message' 
    => 'Please enter your email!']]]; 
        // error to be displayed in amp - mustache
        header("Content-Type:  application/json");
        echo json_encode($error_array); 
        header("HTTP/1.1 400 Bad Request");
        return false;
    }
    /*
        Checking of the email is valid, i.e. if it contains
        an '@' and at least one dot (i.e. example@example.com)
    */ 
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)){
          $error_array = ['verifyErrors'
          =>[['name' => 'Invalid email','message' 
          => 'Enter a valid email!']]]; 
        // error to be displayed in amp - mustache
        header("Content-Type:  application/json");
        echo json_encode($error_array); 
        header("HTTP/1.1 400 Bad Request");
        return false;
        
    }
/*
    Checking whether the user hasn't already
    subscribed with this email
 */
     if(in_array( $email, $our_subscribers) ) {
       
        $error_array = ['verifyErrors'=>[['name' => 
        'Email exists','message'
         => 'Email already exists in our database.']]]; 
        // error to be displayed in amp - mustache
        header("Content-Type:  application/json");
        echo json_encode($error_array); 
        header("HTTP/1.1 400 Bad Request");
        return false;
    }
// If there are no errors, submit the form. 
// Below we are pretending we are adding 
// the submitted email in the DB
if(1 == 1)
{
   $data = ['submit-sucess'=>'Success!'];
  echo json_encode($data);
  header("HTTP/1.1 200 OK");
}