Search into website societe.com if a SIRET exists or not. Other solutions exists like with http://ec.europa.eu/taxation_customs/index_fr.htm

check_siret.php

<?php

/*
 * Search into website societe.com if a SIRET exists or not
 */
function SIRETExists($checkSIRET)
{
  $checkRCS = substr($checkSIRET, 0, 9);
  $retURL = @file_get_contents("http://www.societe.com/cgi-bin/recherche?rncs=".$checkRCS);
  if (empty($retURL))
    return FALSE;

  if (preg_match("/Erreur de formulaire./", $retURL))
    return FALSE;
  else
  {
    preg_match("/SIRET.*([0-9]{14})/ms", $retURL, $matches);
    if ($checkSIRET === $matches[1])
      return TRUE;
    else
      return FALSE;
  }
}

/* Test part */
echo SIRETExists("0123456789")."\n";
echo SIRETExists("78861862700012")."\n";
echo SIRETExists("75130154000017")."\n";

?>