PHP symbol "<<<"

I found the following code in a tutorial on building a RSS feed. I can’t find a reference to the “<<<” on the PHP manual web site. Would someone point me to the documentation on this symbol.

<?
$content = <<<CONTENT
<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0">
        <channel>
                <title>$chan_title</title>
                <link>$chan_link</link>
                <description>$chan_desc</description>
                <image>
                        <title>$record_num</title>
                        <url>$image_url</url>
                        <link>$chan_link</link>
                </image>
$items
        </channel>
</rss>

CONTENT;