请教这个结构，怎么用php, DOM读

请问这个结构，如何用php, DOM读？
<a   id= "1 ">
    <a   id= "2 ">
        <a   id= "3 "   />
    </a>
    <a   id= "2 ">
        <a   id= "4 "   />
    </a>
</a>

所有的标签名都是a,一样。如何列出有多少个是id=2的a。用getelementbyname，就都列出来了。请问如何写   php?

[解决办法]
$doc = new DOMDocument();
$doc-> load( 'tttt.xml ' );

$xpath = new DOMXPath($doc);
$entries = $xpath-> evaluate( '//a[@id=2] ');
foreach ($entries as $e){
echo $e-> getAttribute( 'id ');
}
[解决办法]
$doc = new DOMDocument();
$doc-> load( 'tttt.xml ' );

$xpath = new DOMXPath($doc);
$entries = $xpath-> evaluate( '/a/a ');
foreach ($entries as $e){
echo $e-> getAttribute( 'id ');
}