使用PDFLib绘制矩形

我正在寻找一个在现有pdf文件中绘制矩形的解决方案。 我试图用函数rect(x,y,width,height)来实现这一点,但它不起作用。我收到一条错误消息,说“不能在'object'scope'中调用函数”,但我不知道如何正确。

$searchpath = dirname(dirname(__FILE__)) . '/data';
$pdfinput = "2972172dpi.pdf";
$docoptlist = "requiredmode=minimum";

try {
$p = new PDFlib();
$p->set_option("errorpolicy=return");

# all strings are expected as utf8
$p->set_option("stringformat=utf8");
$p->set_option("SearchPath={{" . $searchpath . "}}");
$doc = $p->open_pdi_document($pdfinput, $docoptlist);
if ($doc == 0) {
    die("Error: " . $p->get_errmsg());
}
$pcosmode = $p->pcos_get_number($doc, "pcosmode");
if ($pcosmode == 0) {
    printf("Minimum mode: no more information available<br/><br/>");
    $p->delete();
    exit(0);
} else {
    $pages = $p->pcos_get_number($doc, "length:pages");
    printf($pages . " pages: <br/>");
    for ($page = 0; $page < $pages; $page++) {
        $param = "pages[" . $page . "]/";
        $widht = round($p->pcos_get_number($doc, $param . "width") / 2.8346456692913);
        $height = round($p->pcos_get_number($doc, $param . "height") / 2.83464566929134);
        printf("Page " . ($page + 1) . " : largeur= %d mm - hauteur= %d mm<br/>", $widht, $height);
        if ($widht == 210 && $height == 297) {
          $p->rect(8.503937007874, 8.503937007874, 578.2677165354, 824.8818897638);
          $p->stroke();
          echo 'Dessiné';
          } */
    }
}
$p->close_pdi_document($doc);
} catch (PDFlibException $e) {
die("PDFlib exception occurred in starter_pcos sample:<br/>" .
        "[" . $e->get_errnum() . "] " . $e->get_apiname() . ": " .
        $e->get_errmsg() . "<br/>");
} catch (Exception $e) {
die($e);
}