I found the answer to my problem, actually a colleague of mine did !
The curlopt_returntransfer was returning a document deprived of all text. So he added this code to make sure that the function would return all of the text included in the page.
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $URL);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_VERBOSE, true);
curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/5.0 (Windows NT 6.1; rv:27.0) Gecko/20100101 Firefox/27.0');
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
$resultat = curl_exec($ch);
curl_close($ch);