Simple Contact Form API

contact_form_api.php

<?php

# ContactFormAPI
#
# Simple script with PHP class to send via e-mail the details
# of a form submitted to it via HTTP POST. PHP 5.5+ required.
#
# <form action="/contact_form_api.php" method="POST" accept-charset="UTF-8">
#   <input name="utf8" type="hidden" value="✓">
#   <!-- ... -->
# </form>
#
# This is nothing big or intensive. Not everything was tested
# rigurously. There is no validation of the fields except for
# the redirect URL. No spam prevention. USE IT AT YOUR OWN RISK.
#
# License: MIT

class ContactFormAPI {
  const EMAIL_TO = 'your.address@example.com';
  const REDIRECT_DOMAIN = 'example.com';
  const SUBJECT_DEFAULT = 'Contact Form';
  const SUBJECT_FORMAT = '[SITE] %s';
  const TIME_FORMAT = 'Y-m-d H:i:s e';
  const TIME_ZONE = 'GMT';

  function email_to() { return self::EMAIL_TO; }
  function redirect_domain() { return self::REDIRECT_DOMAIN; }
  function subject_default() { return self::SUBJECT_DEFAULT; }
  function subject_format() { return self::SUBJECT_FORMAT; }
  function time_format() { return self::TIME_FORMAT; }
  function time_zone() { return self::TIME_ZONE; }

  function __construct($fields = array()) {
    $this->fields = $fields;
  }

  function current_time() {
    $date = new DateTime('now', new DateTimeZone($this->time_zone()));
    return $date->format($this->time_format());
  }

  function mail() {
    return mail($this->email_to(), $this->mail_subject(),
      $this->mail_body());
  }

  function mail_body() {
    $output = '';

    foreach($this->fields as $key => $value) {
      $output .= sprintf("%s:\n%s\n\n", $key, $value);
    }

    $output .= sprintf("IP:\n%s\n\n", $_SERVER['REMOTE_ADDR']);
    $output .= sprintf("Referrer:\n%s\n\n", $_SERVER['HTTP_REFERER']);
    $output .= sprintf("Time:\n%s\n\n", $this->current_time());

    return $output;
  }

  function mail_subject() {
    $subject = $this->fields['subject'] ?: $this->subject_default();
    return sprintf($this->subject_format(), $subject);
  }

  function go_back() {
    return $this->redirect($_SERVER['HTTP_REFERER']);
  }

  function go_forward() {
    return $this->redirect($this->redirect_url());
  }

  function redirect($url) {
    if(!$this->validate_redirect_url($url)) return false;
    header('Location: ' . $url);
  }

  function redirect_url() {
    return $this->fields['redirect_url'] ?: $_SERVER['HTTP_REFERER'];
  }

  function respond() {
    try {
      $this->mail() ? $this->go_forward() : $this->go_back();
    } catch(Exception $e) {
      return $this->response_error($e->getMessage());
    }
  }

  function response_error($msg) {
    http_response_code(500);
    header('Content-Type: application/json');
    echo json_encode(array('error' => $msg));
  }

  function validate_redirect_url($url) {
    $parsed = parse_url($url);
    $domain = $parsed['host'];

    if($domain != $this->redirect_domain()) {
      throw new Exception("Forbidden redirection domain: " . $url);
      return false;
    }

    return true;
  }
}

if(php_sapi_name() != 'cli') {
  $contact_form_api = new ContactFormAPI($_POST);
  $contact_form_api->respond();
}