<?php
$doc = new DOMDocument();
$doc->load( 'things.xml' );
$doc->formatOutput = true;

if( $_POST['clear'] ){
  $thingset = $doc->getElementsByTagName("thingset")->item(0);
  $new = $doc->createElement('thingset');
  $doc->documentElement->replaceChild($new, $thingset);
}else{
  $thing = $doc->createElement('thing');
  $text = $doc->createTextNode(htmlentities($_POST['thing']));
  $thing->appendChild($text);
  $doc->getElementsByTagName("thingset")->item(0)->appendChild($thing);
}

$doc->save( 'things.xml' );

header( 'Location: things.php' );

?>