<?php 
$string = <<<EOT
<?xml version="1.0" encoding="UTF-8"?>
<catalogue id="1">
<progname>Panzani</progname>
<store>
  <name>Panzani Pasta</name>
  <products>
   <product>
    <storeData>
     <data type="modele">Farfale</data>
     <data type="prix_catalogue">3.35</data>
    </storeData>
  </product>
  </products>
</store>
</catalogue>
EOT;
$xml = simplexml_load_string($string);
$data = (string) $xml->store->products->product->storeData->data[0];

echo $data;
?>

Farfale