function api_endpoints() {
  register_rest_route('sendform', '/kontakt/', array(
    'methods' => 'POST',
    'callback' => 'send_contact_form'
  ));
}

add_action( 'rest_api_init', 'api_endpoints')

function send_contact_form( $request ){
  $full_name = sanitize_text_field( trim($request['full_name']) );
  $email = sanitize_email( trim($request['email']) );
  $body = wp_ksest_post( trim($request['body']) );
  $errors = [];
  if ( empty( $full_name)) {
    $errors[] = "Podaj Imię i Nazwisko"
  }
  if ( empty( $email) || ! filter_var( variable: $email, filter: FILTER_VALIDATE_EMAIL)) {
    $errors[] = "Podaj adres E-mail"
  }
  if ( empty( $body)) {
    $errors[] = "Napisz Wiadomość"
  }
  if ( count( $errors )) {
    return new WP_Error( code: 'contact_form_errors', message: $errors, [ 'status' => 422 ])
  }

  $message = "Imię i nazwisko: {$full_name}. <br> Od: {$email}. <br> Wiadomosc: $body";
  $headers = ['Content-Type: text/html; charset=UTF-8']
  wp_mail($email, 'contact_form', $message, $headers)
  return 'Sucess;'
}