$tmp_filename = "Test.xml";

if(file_exists($tmp_filename)){
	$xmldoc = new DOMDocument();
	$xmldoc->load($tmp_filename);
	$root = $xmldoc->documentElement;
}else{
	$xmldoc = new DOMDocument('1.0', 'UTF-8');
	$xmldoc->xmlStandalone = true;
	$root = $xmldoc->createElement("list");
	$root = $xmldoc->appendChild($root);
}
		
$xmldoc->preserveWhiteSpace = false;
$xmldoc->formatOutput = true;
			
$newentry = $xmldoc->createElement("listpoint");
	$newentry->appendChild($xmldoc->createElement("eintrag1", htmlspecialchars($var_eintrag1)));
	$newentry->appendChild($xmldoc->createElement("eintrag2", htmlspecialchars($var_eintrag2)));
	//...

$root->appendChild($newentry);
		
$xmldoc->save($tmp_filename, LIBXML_NOEMPTYTAG);

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<list>
  <listpoint>
    <eintrag1>nur so</eintrag1>
    <eintrag2>was anderes</eintrag2>
  </listpoint>
<listpoint><eintrag1>nur so</eintrag1><eintrag2>was anderes</eintrag2></listpoint></list>