PHP - Cannot modify headers already sent
<p>When trying to run my code, it returns me that: </p>

<blockquote>
  <p>Cannot modify headers already sent</p>
</blockquote>

<pre><code>&lt;?php 

 if(!isset($_SERVER['HTTP_REFERER'])){ 
 header("Location: http://www.example.com/1.php");
 }

?&gt;
</code></pre>