title: Why are some variables in PHP written in uppercase?
body: <p>Why does some code in PHP have to be written in uppercase? </p>
  
  <p>For instance:</p>
  
  <pre><code>if(isSet($_GET['lang']))
  $lang = $_GET['lang'];
  $_SESSION['lang'] = $lang;
  </code></pre>
  
  <p>Do they work if I write them in lowercase?</p>