RSS 源不显示 PHP 变量

>我有一个RSS提要，由于某种原因没有显示我的PHP变量。当返回提要时，它会返回原始标签，例如标题将是时间表$yesterdayd而不是时间表 - 4月10日星期二

<?PHP
 include("../config.php");
 #// Timetable Clearup Variabls
$yesterday = strtotime('yesterday');
$yesterdow = date('l',$yesterday);
$order = "SELECT * FROM timetable WHERE day = '$yesterdow' ORDER BY time";
$result = mysql_query($order);
$yesterdayd = date('F jS, Y', time()-86400);
    //SET XML HEADER
    header('Content-type: text/xml');
    //CONSTRUCT RSS FEED HEADERS
    $output = '<rss version="2.0">';
    $output .= '<channel>';
    $output .= '<title>Timetable - $yesterdayd </title>';
    $output .= '<description>Timetable.</description>';
    $output .= '<link>http://site.com/</link>';
 ###   $output .= '<copyright>Your copyright details</copyright>';
    //BODY OF RSS FEED
   $output .= '<item>';
        $output .= '<title>Timetable for $yesterdayd</title>';
        $output .= '<description>" . htmlspecialchars($row['username']) . "</td><td>" . htmlspecialchars($row['time']) . "</description>';
        $output .= '<link>Link to Item</link>';
        $output .= '<pubDate>Date Published</pubDate>';
   $output .= '</item> ';
    //CLOSE RSS FEED
   $output .= '</channel>';
   $output .= '</rss>';
    //SEND COMPLETE RSS FEED TO BROWSER
    echo($output);
?>

有什么想法吗？