diff --git a/400.php b/400.php
index a95f404..cc56d3e 100755
--- a/400.php
+++ b/400.php
@@ -18,7 +18,7 @@
             <em>grille</em> étant une suite de 81 caractères représentant la grille de gauche à droite puis de haut en bas, soit :
             <ul>
                 <li>un chiffre entre 1 et 9 pour les cases connues</li>
-                <li>un point pour les case vides</li>
+                <li>un tiret (-) pour les case vides</li>
             </ul>
             Exemple :<br/>
             <a href='<?=$newGridUrl?>'><?=$newGridUrl?></a>
diff --git a/classes.php b/classes.php
index 8372a13..349a4d1 100755
--- a/classes.php
+++ b/classes.php
@@ -1,5 +1,5 @@
 <?php
-    const UNKNOWN = ".";
+    const UNKNOWN = "-";
 
     $validGrids = array();
 
@@ -27,15 +27,19 @@
         
     class Box {
         public $values = array('1', '2', '3', '4', '5', '6', '7', '8', '9');
-    
+        public $value = UNKNOWN; 
+        public $rowId;
+        public $columnId;
+        public $regionId;
+        public $candidates;
+        public $candidateRemoved = array();
+        public $neighbourhood = array();
+
         function __construct($rowId, $columnId, $regionId) {
-            $this->value = UNKNOWN;
             $this->rowId = $rowId;
             $this->columnId = $columnId;
             $this->regionId = $regionId;
             $this->candidates = $this->values;
-            $this->candidateRemoved = array();
-            $this->neighbourhood = array();
         }
         
         function searchCandidates() {
@@ -48,8 +52,13 @@
     }
     
     class Grid {
+
+        private $boxes = array();
+        private $rows;
+        private $columns;
+        private $regions;
+
         function __construct($gridStr="") {
-            $this->boxes = array();
             $this->rows = array_fill(0, 9, array());
             $this->columns = array_fill(0, 9, array());
             $this->regions = array_fill(0, 9, array());
diff --git a/index.php b/index.php
index c68f106..191459a 100755
--- a/index.php
+++ b/index.php
@@ -1,19 +1,19 @@
 <?php 
     require("classes.php");
 
-    global $sudokuGridSolutions;
-    if (!isset($sudokuGridSolutions)) $sudokuGridSolutions = array();
-
     $fullUrl = $_SERVER["REQUEST_SCHEME"]."://".$_SERVER["HTTP_HOST"].$_SERVER["DOCUMENT_URI"];
     $dirUrl = dirname($fullUrl);
     $currentGrid = strip_tags($_SERVER['QUERY_STRING']);
 
-    if (preg_match("/^[1-9.]{81}$/", $currentGrid)) {
-        if (!array_key_exists($currentGrid, $sudokuGridSolutions)) {
+    if (preg_match("/^[1-9-]{81}$/", $currentGrid)) {
+        session_id($currentGrid);
+        session_start(["use_cookies" => false]);
+
+        if (!array_key_exists("nbSolutions", $_SESSION)) {
             $grid = new Grid($currentGrid);
-            $sudokuGridSolutions[$currentGrid] = $grid->containsDuplicates() ? -1 : $grid->countSolutions(2);
+            $_SESSION["nbSolutions"] = $grid->containsDuplicates() ? -1 : $grid->countSolutions(2);
         }
-        switch($sudokuGridSolutions[$currentGrid]) {
+        switch($_SESSION["nbSolutions"]) {
             case -1:
                 $warning = "Cette grille contient des doublons.";
                 break;
@@ -27,13 +27,18 @@
         }
         require("sudoku.php");
     } else {
-        $grid = new Grid();
-        $gridAsString = $grid->toString();
-        $newGridUrl = "$dirUrl/?$gridAsString";
-        $sudokuGridSolutions[$gridAsString] = 1;
         if ($currentGrid) {
             require("400.php");
         } else {
+            $grid = new Grid();
+            $gridAsString = $grid->toString();
+            $newGridUrl = "$dirUrl/?$gridAsString";
+    
+            session_id($gridAsString);
+            session_start(["use_cookies" => false]);
+    
+            $_SESSION["nbSolutions"] = 1;
+
             header("Location: $newGridUrl");
         }
     }