$xml=simplexml_load_file('bb.xml');
foreach($xml->item as $v){
      if($v->id==4) echo "id : $v->id<br>pd : $v->pd";
}

------解决方案--------------------

xpath，还有其他答案吗。

PHP code
[User:root Time:10:28:18 Path:/home/liangdong/php]$ php xpath.php 
pd B004
[User:root Time:10:28:19 Path:/home/liangdong/php]$ cat xpath.php 
<?php
$str = <<<EOF
<channel>
  <item>
  <id>1</id>
  <pd>B007</pd>
  </item>
  <item>
  <id>2</id>
  <pd>B001</pd>
  </item>
  <item>
  <id>3</id>
  <pd>B000</pd>
  </item>
  <item>
  <id>4</id>
  <pd>B004</pd>
  </item>
  <item>
  <id>5</id>
  <pd>B003</pd>
  </item>
  </channel>
EOF;

$xml = simplexml_load_string($str, "SimpleXMLElement", LIBXML_NOBLANKS);
$res = $xml->xpath("/channel/item/id[text()=4]/parent::item/pd");
foreach ($res as $node) {
        echo $node->getName() . " " . $node->{0} . PHP_EOL;
}
?>