title: stop explode() expects parameter 2 to be string warning
body: <p>I have a form that's allow the user to select 2 or more categories.</p>
  
  <p>php check url like:</p>
  
  <pre><code> $categories= (!isset($_GET["categories"])?0:explode(";",$_GET["categories"]));
  </code></pre>
  
  <p>normal URL is like this</p>
  
  <pre><code>    site.com/cat.php?page=&amp;categories=1;2
  </code></pre>
  
  <p>if the user change the URL to</p>
  
  <pre><code>    site.com/cat.php?page=&amp;categories[1]
  </code></pre>
  
  <p>the site will show </p>
  
  <pre><code>      Warning: explode() expects parameter 2 to be string, array given in...
  </code></pre>
  
  <p>so I tried to check $_GET like this</p>
  
  <pre><code>     is_string($_GET["category"])
  </code></pre>
  
  <p>but with no luck</p>