generazione codice xml tramite php

[fifth]

Ciao a tutti. In un file php ho inserito la seguente struttura:

echo "<?xml version='1.0' encoding='UTF-8' ?>";
echo "<data id='test'>";
echo "<first_field id ='$first'>";
echo"<second_field id='$second' label='second'>";
echo"<third_field id='$third'>";
echo"<fourth_field value='2006'>";
echo "<result id='01'>'$res'</result>";
echo "<result id='02'>'$res2'</result>";
echo"</fourth_field >";
echo"</third_field>";
echo"</second_field >";
echo"</first_field>";
echo"</data >";

ma mi da l'errore :

Only one top level element is allowed in an XML document. Error processing resource

Come mai?