Problems with special characters in an XML file created with PHP

I have a php script that generates an xml of some tables and it seemed to work correctly, but now one of the values of the fields has a "Ñ" and the browsers take it as an error:

XML read error: malformed

Line number 33, column 36: ARTURO CHAVEZ NUÑ

part of my code is this:

<?php


function crea_fichero($cadena){

    $flujo = fopen('club.xml', 'w');
    fputs($flujo, $cadena);
    fclose($flujo);
}


$opc = array(PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8");
$dsn = "mysql:host=localhost;dbname=prueba_importar";
$usuario = 'root';
$contrasena = '';

$dwes = new PDO($dsn, $usuario, $contrasena, $opc);


if (isset($dwes)){

   $xml="<xml>\n";
   $xml.="\t<club>".date("d/m/Y")."</club>\n";

          $sql2="SELECT * FROM alumnos INNER JOIN (alumnosexamen INNER JOIN examen ON examen.claveexamen=alumnosexamen.claveexamen) ON alumnosexamen.claveweb= alumnos.claveweb AND alumnosexamen.clavealumno=alumnos.clavealumno WHERE examen.estatus='APROBADO' AND alumnos.clavealumno=0 ";


        echo '<br>';

        $cadena="alumnos";

        $xml .= "\t<".$cadena.">\n";
        $result= $dwes->query($sql2);

        $tamanio=strlen($cadena);
        $cadena2=substr($cadena,0,($tamanio-1));

        while($fila = $result->fetch(PDO::FETCH_ASSOC)){
            print_r($fila);

            $xml .= "\t\t<".$cadena2.">\n";

                        $clavealumno=utf8_decode($fila['clavealumno']);
                        $claveweb=utf8_decode($fila['claveweb']);
                        $claveescuela=utf8_decode($fila['claveescuela']);
                        $nombre=utf8_decode( $fila['nombre']);

                        $domicilio=utf8_decode($fila['domicilio']);
                        $ciudad=utf8_decode($fila['ciudad']);
                        $estado=utf8_decode($fila['estado']);
                        $telefono=utf8_decode( $fila['telefono']);

            $xml .= "\t\t</".$cadena2.">\n";
        }// while($fila
        $xml .= "\t</".$cadena.">\n";

    $xml .="</xml>";
crea_fichero($xml);
}//if(isset($dwes))
$result=null;
$dwes=null;

?>