diff --git a/src/Url.php b/src/Url.php
index 5f4dbb0..1e3e89f 100644
--- a/src/Url.php
+++ b/src/Url.php
@@ -531,14 +531,14 @@ class Url {
   /**
    * Add parameter in specified URL
    *
-   * @param string &$url The reference of the URL
+   * @param string $url The URL
    * @param string $parameter The parameter name
    * @param string $value The parameter value
    * @param boolean $encode Set if parameter value must be URL encoded (optional, default: true)
    *
    * @return string The completed URL
    */
-  public static function add_url_parameter(&$url, $parameter, $value, $encode=true) {
+  public static function add_url_parameter($url, $parameter, $value, $encode=true) {
     if (strpos($url, '?') === false)
       $url .= '?';
     else
@@ -550,15 +550,15 @@ class Url {
   /**
    * Add parameters in specified URL
    *
-   * @param string &$url The reference of the URL
+   * @param string $url The URL
    * @param array<string,string> $parameters The parameters as an associative array
    * @param boolean $encode Set if parameters values must be URL encoded (optional, default: true)
    *
    * @return string The completed URL
    */
-  public static function add_url_parameters(&$url, $parameters, $encode=true) {
+  public static function add_url_parameters($url, $parameters, $encode=true) {
     foreach($parameters as $parameter => $value)
-      self :: add_url_parameter($url, $parameter, $value, $encode);
+      $url = self :: add_url_parameter($url, $parameter, $value, $encode);
     return $url;
   }