PHP致命错误:无法重新分配$this

我从脚本中得到这个错误:

[Fri Apr 23 10:57:42 2010] [error] [client 10.0.0.1] PHP Fatal error:  Cannot re-assign $this in C:\\Program Files\\Apache Software Foundation\\Apache2.2\\htdocs\\abp\\fol\\test.php on line 27, referer: http://abp.bhc.com/fol/

下面是产生错误的代码:

<?php
$voiceboxes = array(
    '141133'    => array(
        'title' => 'Title',
        '1'     => array(
            'Title' => 'Title2',
            'Link'  => 'http://...',
        ),
        '12'    => array(
            'Title' => 'Title3',
            'Link'  => 'http://...',
        )
    ),
    '1070453'   => array(
        'title' => 'Title4',
        '1'     => array(
            'Title' => 'Title5',
            'Link'  => 'http://...',
        )
    )
);
$last = 0;
//$this = 0;
echo "<ol>\n";
foreach ($voiceboxes as $key => $value) {
    $last = 0;
    $this = null; //Error is thrown here, Line 27
    //$voiceboxes[$key]['title']
    echo "<ol>\n";
    foreach ($value as $key2 => $value2) {
        if ($key2 == 'title') {
            echo "<li>$value2</li>\n";
        } else {
            $this = (int) $key2;
            if ($this == $last + 1) {
                echo '<li>';
            } else { '<li value="' . $key2 . '">';}
            $last = $key2;
            echo $voiceboxes[$key][$key2]['Title'] . "<br/>" . $voiceboxes[$key][$key2]['Link'] . '</li>' . "\n";
        }
    }
    echo "</ol>\n";
}