title: Autoincrementing strings
body: <p>I'm trying to build a hierarquical list of categories, like this:</p>
  
  <pre><code>1 Category
    1.1 Children
    1.2 Children
        1.2.1 Children
  </code></pre>
  
  <p>Here's my code:</p>
  
  <pre><code>$a = "1.1";
  echo ++$a; // 2.1
  
  $b = "1.1.1";
  echo ++$b; // 1.1.2
  </code></pre>
  
  <p>Why <code>$a</code> increments to <strong>2.1</strong> instead of <strong>1.2</strong> like <code>$b</code>?</p>