<?php
$erreurs = array("A", "B", "C", "D");
$doc = new DomDocument();
$doc->LoadXML('<root/>');
$root = $doc->documentElement;
$errorgroup = $doc->createElement('error');
foreach($erreurs as $erreur) {
    $missinggroup = $doc->createElement('missing', $erreur);
    $errorgroup->appendChild($missinggroup);
}
$root->appendChild($errorgroup);
echo $doc->saveXml();

<?xml version="1.0"?>
<root><error><missing>A</missing><missing>B</missing><missing>C</missing><missing>D</missing></error></root>