<?php
class XmlValidator {
    public static function validate($xmlString) {
        $errors = [];
        $dom = new DOMDocument();
        if (!@$dom->loadXML($xmlString)) {
            $errors[] = "Failed to parse XML.";
            return $errors;
        }

        $rootElement = $dom->documentElement;
        if ($rootElement->tagName !== 'ectd:ectd') {
            $errors[] = "Root element is not 'ectd:ectd'.";
        }

        $dtdVersion = $dom->getElementsByTagName('dtd-version')->item(0);
        if (!$dtdVersion || empty($dtdVersion->nodeValue)) {
            $errors[] = "Required tag 'dtd-version' is missing or empty.";
        }

        $submissionType = $dom->getElementsByTagName('submission-type')->item(0);
        if (!$submissionType || empty($submissionType->nodeValue)) {
            $errors[] = "Required tag 'submission-type' is missing or empty.";
        }
        
        return $errors;
    }
}
?>