Php 第1列第1行的XML错误：文档为空

我正在尝试从MySQL导入PHP并粘贴到XML中，这就产生了这个错误

function get_info(){
    global $conn;
     $sql ="SELECT * FROM auto ORDER BY evento_id DESC LIMIT 1";
     $result = $conn->query($sql);

     $xml = new DOMDocument();
     $xml->formatOutput = true;
     $r = $xml->createElement( "events" );
     $xml->appendChild( $r );

     while ($row = $result->fetch_assoc()) {
    
        $b = $xml->createElement( "event" );
             $evento_id = $xml->createElement( "evento_id" );
             $evento_id->appendChild($xml->createTextNode( $row['evento_id']));
             $b->appendChild( $evento_id );
          $r->appendChild( $b );
        }

        mysqli_free_result($result);
        $conn->close();
    
        header('Content-Type: text/xml');
        print $xml->saveXML();
    
        return 0;
}