<?php
    $xml = "<?xml version='1.0' encoding='UTF-8'?>
<test>
  <p>
    <tag2>
      <tag3>test2</tag3>
    </tag2>
  </p>
  <tag2>
    <tag3>test2</tag3>
  </tag2>
  <beg id='1'/>
  <p>
    <tag2>test2</tag2>
  </p>
  <tag2>
    <tag3>test2</tag3>
  </tag2>
  <p>
    <tag2>
      <tag3>test2</tag3>
    </tag2>
    <tag2>
      <tag3>test2</tag3>
    </tag2>
    <end id='1'/>
    <tag2>
      <tag3>test2</tag3>
    </tag2>
  </p>
  <beg id='2'/>
  <tag2>
    <tag3>test2</tag3>
  </tag2>
  <end id='2'/>
  <p />
</test>";
    $dom =  new  DomDocument();
    $dom->loadXML($xml);
    $xpath =  new  DomXPath($dom);
   
    $tag1 = $dom->getElementsByTagName("beg")->item( 0 );
     echo  $xpath->query("following::tag2/tag3[following::end/@id=1]", $tag1)->length;
?>