高手求教，PHP怎么向xml中插入新信息？

应该是这样：
$doc = new DOMDocument();
$doc->load( "pic.xml");
//这里你要获取他的第一个tag，因为getElementsByTagName获取的结果是个数组对象
$folder = $doc->getElementsByTagName("folder") -> item(0);

$new_file=$doc->createElement("file");

$pname = $doc -> createAttribute('pname');
$pclass = $doc -> createAttribute('class');
$pcar = $doc -> createAttribute('pcar');

$newspname = $doc -> createTextNode($name);
$newspclass = $doc -> createTextNode($_POST['Sclass']);
$newspcar = $doc -> createTextNode($_POST['Scar']);

$pname -> appendChild($newspname);
$pclass -> appendChild($newspclass);
$pcar -> appendChild($newspcar);

$new_file -> appendChild($pname);
$new_file -> appendChild($pclass);
$new_file -> appendChild($pcar);

$folder -> appendChild($new_file);
//$doc ->appendChild($folder);//前面已经append了，就不需要在append了

$doc -> save("pic.xml");