Parse XML by Category

gistfile1.php

<?php
    $jobs = simplexml_load_file('jobfeed.xml');

	function output_list($jobs = array())
	{
	    $data = array();

	    foreach($jobs->xpath('/jobs/job') as $job_details)
	    {
	        $category = $job_details->department;
	        $title    = $job_details->title;
	        $url      = $job_details->url;

	        if(!isset($data[$category]))
	        {
	            $data[$category] = array();
	        }

	        $data[$category][] = '<li>Category: '.$category.'<br>Title: '.$title.'<br>URL: '.$url.'</li>';
	    }

	    foreach($data as $category => $items)
	    {
	        $data[$category] = '<li><ul>'.implode('', $items).'</ul></li>';
	    }

	    return '<ul>'.implode('', $data).'</ul>'; 
	}

	echo output_list($jobs);
?>

var_dump($data); gives me this: array(0) { }

var_dump($category); gives me this: object(SimpleXMLElement)#47 (0) { }

Both would imply that there is no data being returned for the $category variable, which confuses the hell out of me because if I do this:

<ul>
<?php
    $jobs = simplexml_load_file('jobfeed.xml');

    foreach ($jobs->xpath('/jobs/job') as $jobDetails) {
        $category = $jobDetails->department;
        $title = $jobDetails->title;
        $url = $jobDetails->url;

        echo '<li>Category: ',$category,'<br>Title: ',$title,'<br>URL: ',$url,'</li>';
    }
?>
</ul>

The categories are output perfectly fine.