Validate XML using a custom DTD in PHP

Question

Is there a way (without installing any libraries) of validating XML using a custom DTD in PHP?

Answer 1

My interpretation of the original question is that we have an "on board" XML file that we want to validate against an "on board" DTD file. So here's how I would implement the "interpolate a local DTD inside the DOCTYPE element" idea expressed in comments by both Soren and PayamRWD:

public function validate($xml_realpath, $dtd_realpath=null) {
    $xml_lines = file($xml_realpath);
    $doc = new DOMDocument;
    if ($dtd_realpath) {
        // Inject DTD inside DOCTYPE line:
        $dtd_lines = file($dtd_realpath);
        $new_lines = array();
        foreach ($xml_lines as $x) {
            // Assume DOCTYPE SYSTEM "blah blah" format:
            if (preg_match('/DOCTYPE/', $x)) {
                $y = preg_replace('/SYSTEM "(.*)"/', " [\n" . implode("\n", $dtd_lines) . "\n]", $x);
                $new_lines[] = $y;
            } else {
                $new_lines[] = $x;
            }
        }
        $doc->loadXML(implode("\n", $new_lines));
    } else {
        $doc->loadXML(implode("\n", $xml_lines));
    }
    // Enable user error handling
    libxml_use_internal_errors(true);
    if (@$doc->validate()) {
        echo "Valid!\n";
    } else {
        echo "Not valid:\n";
        $errors = libxml_get_errors();
        foreach ($errors as $error) {
            print_r($error, true);
        }
    }
}

Note that error handling has been suppressed for brevity, and there may be a better/more general way to handle the interpolation. But I have actually used this code with real data, and it works with PHP version 5.2.17.