$string = "
<table>
    <tr>
        <td> 1 </td>
        <td> 2 </td>
        <td> 3 </td>
    </tr>
</table>
<table>
    <tr>
        <td> 4 </td>
        <td> 5 </td>
        <td> 6
            <table>
                <tr>
                    <td> A </td>
                    <td> B </td>
                    <td> C </td>
                </tr>
                <tr>
                    <td> D </td>
                    <td> E </td>
                    <td> F </td>
                    <td> G </td>
                    <td> H </td>
                </tr>
            </table>
        </td>
    </tr>
    <tr>
        <td> 7 </td>
        <td> 8 </td>
        <td> 9 </td>
    </tr>
</table>";

$doc = new DOMDocument();
$doc->preserveWhiteSpace = false;
$doc->loadHTML($string);
$i=0;

while(is_object($table = $doc->getElementsByTagName("tr")->item(3))) {
  foreach($table->childNodes as $nodename) {
    if($nodename->nodeName=='td') {
      foreach($nodename->childNodes as $subNodes) {
        if ( $nodename->childNodes->item(4) ) {
          $subNodes->removeThisNode();
        }
      }
    }
  }
}