[User:root Time:00:07:19 Path:/home/liangdong/php]$ php xpath.php 
type:small
color:white
age:5
[User:root Time:00:07:19 Path:/home/liangdong/php]$ cat xpath.php 
<?php
$str = <<<EOF
<?xml version="1.0" encoding="utf8" ?>
<pets>
<pet id="01">
  <type resource="big"/>
  <color resource="black"/>
  <age resource="2"/>
</pet>
<pet id="02">
  <type resource="small"/>
  <color resource="white"/>
  <age resource="5"/>
</pet> 
</pets>
EOF;

$xml = simplexml_load_string($str, "SimpleXMLElement", LIBXML_NOBLANKS);
$res = $xml->xpath("/pets/pet[type[@resource='small'] and color[@resource='white'] and age[@resource='5']]");
foreach ($res as $node) { 
        $children = $node->children();
        foreach ($children as $child) {
                echo $child->getName() . ":" . $child['resource'] . PHP_EOL;
        }
}
?>

------解决方案--------------------
PHP code
$str = <<<EOF
<?xml version="1.0" encoding="utf8" ?>
<pets>
<pet id="01">
  <type resource="big"/>
  <color resource="black"/>
  <age resource="2"/>
</pet>
<pet id="02">
  <type resource="small"/>
  <color resource="white"/>
  <age resource="5"/>
</pet> 
</pets>
EOF;

$xml = simplexml_load_string($str);
$res = $xml->xpath("/pets/pet[type[@resource='small'] and color[@resource='white'] and age[@resource='5']]");

echo $res[0]->attributes()->id; //02