<?php
    $xml = <<<XML
<Klassen>
  <Klas>
     <Klas>HT1</Klas>
     <Omschrijving>Klas HT1</Omschrijving>
  </Klas>
  <Klas>
     <Klas>HT2</Klas>
     <Omschrijving>Klas HT2</Omschrijving>
  </Klas>
</Klassen>
XML;

    $xmlDoc = new DOMDocument();
    $xmlDoc->loadXML($xml);

    $xpath = new DOMXpath($xmlDoc);
    $hallo = $elements = $xpath->query("//Klas/Klas");

    foreach ($hallo as $book) {
        $result = $book->nodeValue;
        echo '<option value="'.$result.'">'.$result.'</option>';
    }

<option value="HT1">HT1</option><option value="HT2">HT2</option>