title: Do not access superglobal $_GET array directly
body: <p>I'm new to PHP, so forgive me if this is a stupid question but why should I not access items in the superglobal arrays directly? NetBeans warns me not to (see title), and I've read the same elsewhere -- however I haven't come across a good explanation yet.</p>
  
  <p>What can go wrong? Is it a security concern, like SQL injection? Or something else?</p>
  
  <p>There are several similar questions on StackOverflow, but none of the answers actually explain what the problem is: they just suggest to use <code>filter_input()</code>. This doesn't satisfy me, since I like knowing how things work. Any explanation will be appreciated. </p>