XML Parsing Error: SYSTEM or PUBLIC, the URI is missing [closed]

// $xml_text is filled with the contents read from the URL
$xml_parser = xml_parser_create();
$res = xml_parse($xml_parser, $xml_text);

if (!$res) {
  $error =
    xml_error_string(xml_get_error_code($xml_parser)).
    " at line ".
    xml_get_current_line_number($xml_parser);
}

// $error contains: "SYSTEM or PUBLIC, the URI is missing at line 1"