方法一
        $doc = new DOMDocument('1.0', 'utf-8');

        $doc->formatOutput = true;

        $rootEle = $doc->createElement('root');

        $doc->appendchild($rootEle);

        $descriptionEle = $doc->createElement('description');

        $rootEle->appendChild($descriptionEle);

        $couponNameEle = $doc->createElement('couponName');

        $couponDescriptionEle = $doc->createElement('couponDescription');

        $couponNameEle->appendChild($doc->createTextNode('jiuzhe'));

        $couponDescriptionEle->appendchild($doc->createTextNode('jiuzhedescription'));

        $descriptionEle->appendchild($couponNameEle);

        $descriptionEle->appendChild($couponDescriptionEle);

        $detailEle = $doc->createElement('detail');

        $rootEle->appendchild($detailEle);

        $itemEle = $doc->createElement('item');

        $detailEle->appendChild($itemEle);

        $codeEle = $doc->createElement('code');

        $codeEle->appendchild($doc->createTextNode('666666'));

        $itemEle->appendchild($codeEle);

        $itemEle = $doc->createElement('item');

        $detailEle->appendChild($itemEle);

        $codeEle = $doc->createElement('code');

        $codeEle->appendchild($doc->createTextNode('999999'));

        $itemEle->appendchild($codeEle);

        $doc->save('/home/susie/workspace/' . 'offer-coupon.xml');

<root>
　　<description>
　　　　<couponName>jiuzhe</couponName>
　　　　<couponDescription>jiuzhedescription</couponDescription>
　　</description>
　　<detail>
　　　　<item>
　　　　　　<code>666666</code>
　　　　</item>
　　　　<item>
　　　　　　<code>999999</code>
　　　　</item>
　　</detail>
</root>

$lineFeed = "\n";

$xml = '<?xml version="1.0" encoding="utf-8"?>' . $lineFeed;

$xml .= '<root>' . $lineFeed;

$xml .= '<description>' . $lineFeed;

$xml .= '<couponName>' . "1111" . '</couponName>' . $lineFeed;

$xml .= '<couponDescription>' . "222" . '</couponDescription>' . $lineFeed;

$xml .= '<quantity>' . "3333" . '</quantity>' . $lineFeed;

$xml .= '</description>' . $lineFeed;

$xml .= '<codes>' . $lineFeed;

$xml .= '<code>' . "codecode" . '</code>' . $lineFeed;

$xml .= '</codes>' . $lineFeed;

$xml .= '</root>' . $lineFeed;

header("Content-Type:text/xml charset=UTF-8");

header("Content-Disposition:attachment; filename=offer-coupon.xml");

header('Cache-Control:must-revalidate,post-check=0,pre-check=0');

header('Expires:0');

header('Pragma:public');

$fp = fopen('php://output', 'a');

fwrite($fp, $xml);

fclose($fp);

die();