php-rss重复项

我有这样的RSS:

<title>Session 1</pubDate>
<category>Blue</category>
<category>Green</category>
<evnet:starttime>Mon, 15 Mar 2010 12:05:00 GMT</evnet:starttime>
<evnet:endtime>Mon, 15 Mar 2010 12:30:00 GMT</evnet:endtime>

我正在使用 喜鹊RSS解析器 要获取RSS源:

<?php require_once('magpie/rss_fetch.inc');

$rss = fetch_rss('http://live.visitmix.com/Sessions/RSS');

foreach ($rss->items as $item) { 
    $cat = $item['category'];
    $title = $item['title'];
        echo '<li class="'.$cat.'">'.$title.'</li>';
}


?>

结果是:

<li class="BlueGreen">Title</li>

我要求:

<li class="Blue Green">Session 1</li>

注意类与类之间的间距

如何在类别值之间插入空格?