function getSignature ($params,$key)
{
    /**
     *Function that computes the signature.
     * $params : table containing the fields to send in the payment form.
     * $key : TEST or PRODUCTION key
     */
    //Initialization of the variable that will contain the string to encrypt
    $signature_content = "";
    //sorting fields alphabetically
    ksort($params);
    foreach($params as $name=>$value){
        //Recovery of vads_ fields
        if(substr($name,0,5)=='vads_'){
            //Concatenation with "+"
            $signature_content.= $value."+";
         }
    }
    //Adding the key at the end
    $signature_content.= $key;
    //Encoding base64 encoded chain with SHA-256 algorithm
    $signature = base64_encode(hash_hmac('sha256',$signature_content, $key, true));
    return$signature;
 }