Sending the HTTP Response Start Line in PHP

Using the header function, you can also send the start line of the HTTP response. This is typically used to specify the status. Mainly for the 404 error:

php<?php
  header('HTTP/1.1 404 Not Found');
?>

Задание 1

Complete the code so that it sends the 404 error header:

php<?php
  $arr = ['a', 'b', 'c'];
  $key = $_GET['key'];
  
  if (isset($arr[$key])) {
    echo $arr[$key];
  } else {
    // send 404
    echo 'error';
  }
?>