title: What exactly does the PHP function "test_input()" do?
body: <p>I know I will be voted down for this, but I need to ask. What does the code below mean? Specifically,
  I am confused by  </p>
  
  <pre><code>if (empty($_POST["comment"])) { $comment = ""; 
  </code></pre>
  
  <p>Does this mean that if the user has not inserted his comments, then the field for which the value is "$comment" will be blank? And if the user HAS inserted a comment, then the value "$comment" will be put through the test_input function? I have looked at the w3schools website but I am none the wiser. It is unbelievable that NO website even describes what test_input does, and believe me, I've looked at dozens of websites. In fact, I am not the only SO user who thinks that test_input is a mysterious function.</p>
  
  <pre><code>if ($_SERVER["REQUEST_METHOD"] == "POST") {
      echo 'posted';
      if (empty($_POST["comment"])) {
          $comment = "";
      } else {
          $comment = test_input($_POST["comment"]);
      }
  </code></pre>