please explain this PHP code "<<<CSS"
<blockquote>
  <p><strong>Possible Duplicate:</strong><br>
  <a href="http://stackoverflow.com/questions/3737139/reference-what-does-this-symbol-mean-in-php">Reference - What does this symbol mean in PHP?</a><br>
  <a href="http://stackoverflow.com/questions/6062830/what-is-the-name-for-the-operator">What is the name for the &ldquo;&lt;&lt;&lt;&rdquo; operator?</a>  </p>
</blockquote>



<p>I can infer what this is doing but I need to know how, and possible find some documentation.
The expression in the title <code>&lt;&lt;&lt;CSS</code> seems to set up a block of css to be inserted into a template. However there are no functions to strip the <code>&lt;&lt;&lt;CSS</code> from these text strings yet they do not appear. This leads me to believe that <code>&lt;&lt;&lt;</code> is not just a random marker but has special PHP meaning. Please help.</p>

<pre><code>if(!empty($background['header_image'])){
    $header_image = &lt;&lt;&lt;CSS
background-image: url('{$background['header_image']}');
background-repeat: {$background['header_repeat']};
background-position: top {$background['header_position_x']};
background-attachment: scroll;
CSS;`
</code></pre>
