sanitize.php

<?php
function get_param($key, $filter=FILTER_SANITIZE_STRING) {
 if (!isset($_GET[$key])) {
    return null;
 } 
 
 return filter_var($_GET[$key], $filter);
}

function post_param($key, $filter=FILTER_SANITIZE_STRING) {
 if (!isset($_POST[$key])) {
    return null;
 } 
 
 return filter_var($_POST[$key], $filter);
}

function cookie_param($key, $filter=FILTER_SANITIZE_STRING) {
  if (!isset($_COOKIE[$key])) {
    return null
  }
  
  return filter_var($_COOKIE[$key], $filter);
}