Answered Need help for a snippet which retrieve values on a page and make calculation with them

<form method="post" action="https://myurl.com/">  
  <input type="hidden" name="nospam" value="" />
  <input type="hidden" name="vads_site_id" value="5932111">
  <input type="hidden" name="vads_ctx_mode" value="TEST">
  <input type="hidden" name="vads_trans_id" value="170810203408011">
  <input type="hidden" name="vads_trans_date" value="10/08/17 20:34:08">
  <input type="hidden" name="vads_currency" value="EUR">
  <input type="hidden" name="vads_action_mode" value="INTERACTIVE">
  <input type="hidden" name="vads_page_action" value="PAYMENT">
  <input type="hidden" name="vads_version" value="V2">
  <input type="hidden" name="vads_payment_config" value="SINGLE">
  <input type="hidden" name="signature" value="[[CalculateSignature]]" /> 

  <label>First name</label> 
  <input type="text" name="vads_cust_first_name" value="My first name" readonly />

  <label>Last name</label>  
  <input type="text" name="vads_cust_last_name" value="My last name" readonly />
    
  <input type="submit" value="Go" name="go">
</form>

// Function to calculate signature
// ------------------------------

// Key certificat
$key = "1122334455667788";

// $params : array which contain the fields to send by the form
function getSignature($params){
 
//Initialisation of the variable which contain the string to be encrypted
$contenu_signature = "" ;
 
    // sort the fields by alphabetical order
    ksort($params);
        foreach ($params as $nom =>$valeur)
    { 
    // Retreive the vads_  fields
        if (substr($nom,0,5)=='vads_') { 
 
            // Concatenation with "+" separator
            $contenu_signature .= $valeur."+";
        }
    }
 
// Adding the certificat on the end
$contenu_signature .= $key;
 
// SHA-1 algorythm application
$signature = sha1($contenu_signature);
return $signature ;
}

$string = "INTERACTIVE+TEST+EUR+PAYMENT+10/08/17 20:34:08+"170810203408011+SINGLE+5932111+V2+1122334455667788";
$signature = sha1($string);
$modx->setPlaceholder('signature', $signature);

[[+signature]]

[[!FormItRetriever]]

<form method="post" action="https://myurl.com/">  
  <input type="hidden" name="nospam" value="" />
  <input type="hidden" name="vads_site_id" value="5932111">
  <input type="hidden" name="vads_ctx_mode" value="TEST">
  <input type="hidden" name="vads_trans_id" value="170810203408011">
  <input type="hidden" name="vads_trans_date" value="10/08/17 20:34:08">
  <input type="hidden" name="vads_currency" value="EUR">
  <input type="hidden" name="vads_action_mode" value="INTERACTIVE">
  <input type="hidden" name="vads_page_action" value="PAYMENT">
  <input type="hidden" name="vads_version" value="V2">
  <input type="hidden" name="vads_payment_config" value="SINGLE">
  <input type="hidden" name="signature" value="[[!CalculateSignature]]" /> 
 
  <label>First name</label> 
  <input type="text" name="vads_cust_first_name" value="My first name" readonly />
 
  <label>Last name</label>  
  <input type="text" name="vads_cust_last_name" value="My last name" readonly />
     
  <input type="submit" value="Go" name="go">
</form>

/* CalculateSignature snippet */
//Initialisation of the variable which contain the string to be encrypted
$contenu_signature = "" ;
  
    $params = $modx->getPlaceholders();
    // sort the fields by alphabetical order
    ksort($params);
        foreach ($params as $nom =>$valeur)
    { 
    // Retreive the vads_  fields
        if (strpos($nom,'vads_') !== false)) { 
  
            // Concatenation with "+" separator
            $contenu_signature .= $valeur."+";
        }
    }
  
// Adding the certificat on the end
$contenu_signature .= $key;
  
// SHA-1 algorythm application
$signature = sha1($contenu_signature);
return $signature ;