Please read how DOM works. There are Textnodes between your Elements, 
which also count as items, so ->item(1) would be the a element in your 
example and item(3) the b element.

I'd advise you to use xpath for much more accurate pinpointing of 
nodes...

thank you for your email,help me so much
but i also have question
that how to read the xml

<root><item><a>11</a><b>1</b><c>0</c></item></root>
------->
this xml
i can do the pro

$dom = new DOMDocument();
$dom->load('test.xml');
$root = $dom->documentElement;
$items = $root->childNodes;
$item = $items->item(0);
$itemChildren = $item->childNodes;
$b = $itemChildren->item(1);
echo $b->nodeValue;

but like this xml------->
<root>
<item>
<a>11</a>
<b>1</b>
<c>0</c>
</item>
</root>

if i read $b = $itemChildren->item(1) or item(3)
the php report that:
call to a member function item() on a non-object in .....

please help me how to write the function to read the xml,thank you very much