Problème avec les checkbox

<!DOCTYPE html>    
<html>    
 <head>    
  <meta charset="utf-8" />    
  <title>test</title>    
  
 </head>    
     
 <body>    
  <form>    
   <p>    
    Cochez les aliments que vous aimez manger :    
    <br />    
    <input type="checkbox" name="frites" id="frites" onclick=tout() />    
    <label for="frites">Frites</label>    
    <br />    
    <input type="checkbox" name="steak" id="steak" />    
    <label for="steak">Steak haché</label>    
    <br />    
    <input type="checkbox" name="epinards" id="epinards" />    
    <label for="epinards">Epinards</label>    
    <br />    
    <input type="checkbox" name="huitres" id="huitres" />    
    <label for="huitres">Huitres</label>    
   </p>    
   </form>    
   <script type="text/javascript">    
    function tout() {    
     var input = document.getElementsByTagName("input");    

     var c = input.length;    

     if(input[0].checked) {    
      for(var i = 0; i < c; i++) {    
       if(input[i].type == "checkbox") {    
        input[i].checked = true;    

       }    
      }    
     } else {    
      for(var i = 0; i < c; i++) {    
       input[i].checked = false;    
      }    
     }    
    }    
   </script>    
 </body>    
</html>    
</body>    
</html> 

<!DOCTYPE html> 
<html> 
 <?php 
 session_start(); 
 $disabled = null; 
 ?> 
 <head> 
  <title>Mini-projet</title> 
  <link rel="stylesheet" type="text/CSS" href="mini-projet.css"/> 
 </head> 
 <body> 
  <div id='conteneur'> 
   <div id="menu1"> 
    <div class='button1'> 
     <a href='logout.php'>déconnexion</a> 
    </div> 
    <br> 
    <br> 
    <br> 
    <br> 
    <br> 
    <a href='admin.php'> Accueil</a> 
    <br> 
    <br> 
    <a href='comptesE.php'> Espace Etudiant</a> 
    <br> 
    <br> 
    <a href='comptesP.php'> Espace Enseignant</a> 
    <br> 
    <br> 
    <a href='comptesR.php'> Espace Responsable</a> 
    <br> 
    <br> 
    <a href='comptesS.php'> Espace Secrétaire</a> 
    <br> 
    <br> 
    <a href='comptesA.php'> Espace Administrateur</a> 
    <br> 
    <br> 
   </div> 
   <div id="header"></div> 
   <div id="menu2"> 
    <img src="notest.png"> 
   </div> 
   <div id="body"> 
    <h1 align='center' ><i> Bienvenue <?php echo $_SESSION["uti"] 
    ?></font></i></h1> 
    <br> 
    <br> 
    <br> 
    <form method='POST' action='tableE.php'> 
     <input type='submit' value='mofidier' name='modifier' /> 
     <input type='submit' value='supprimer' name='supprimer' /> 
     <?php 
     if (isset($_POST['modifier']) && !empty($_POST['modifier'])) 
      echo "<input type='submit' value='confirmer' name='ok' align='right'>"; 
     ?> 

     <table width='500px' border='2'> 
      <tr> 
       <td> 
       <input type="checkbox" name="all" id="all" onclick=tout() /> 
       </td> 
       <td>CNE</td> 
       <td>Nom</td> 
       <td>Prenom</td> 
      </tr> 
      <?php 

      $connect = mysql_connect('localhost', 'root', ''); 
      mysql_select_db('gestiondesnotes'); 

      $req = mysql_query("select CNE,Nom,prenom from etudiant"); 

      // afficher tous les utilisateurs de la base de données 
      while ($ligne = mysql_fetch_row($req)) { 
       $Id = $ligne[0]; 
       $nom = $ligne[1]; 
       $prenom = $ligne[2]; 
       if (isset($_POST['modifier']) && !empty($_POST['modifier']) && isset($_POST[$Id])) { 
        $disabled = ""; 

       } else {$disabled = "disabled"; 
       } 

       echo "<tr>"; 
       echo "<td><input type=\"checkbox\" name=" . $Id . " id=" . $Id . " />"; 
       echo "<td>" . $Id . "</td>"; 
       echo "<td><input type='text' name=" . $Id . '-nom' . " value=" . $nom . " " . $disabled . "></td>"; 
       echo " <td><input type='text' name=" . $Id . '-prenom' . " value=" . $prenom . " " . $disabled . "></td>"; 
       echo "</tr>"; 

       // modification des données la base de données 
       if (isset($_POST['ok']) && !empty($_POST['ok'])) { 
        if (isset($_POST[$Id . '-nom']) && !empty($_POST[$Id . '-nom']) && isset($_POST[$Id . '-prenom']) && !empty($_POST[$Id . '-prenom'])) { 
         $n = $_POST[$Id . '-nom']; 

         $p = $_POST[$Id . '-prenom']; 

         $req2 = mysql_query("UPDATE etudiant SET Nom='$n', prenom='$p' WHERE CNE='$Id' "); 
        } 
       } 

       // supprimer des utilisateurs de la base de données 
       if (isset($_POST['supprimer']) && !empty($_POST['supprimer']) && isset($_POST[$Id])) { 

        $req1 = mysql_query("delete from etudiant where CNE=" . $Id . ""); 

        die("suppréssion terminée avec succés"); 
       } 

      } 
      ?> 
     </table> 
    </form> 
   </div> 
   <div id="footer"><img src="header.jpg"> 
   </div> 
  </div> 
  <script type="text/javascript"> 
   function tout() { 
    var input = document.getElementsByTagName("input"); 

    var c = input.length; 

    if(input[3].checked) { 
     for(var i = 0; i < c; i++) { 
      if(input[i].type == "checkbox") { 
       input[i].checked = true; 

      } 
     } 
    } else { 

     for(var i = 0; i < c; i++) { 
      if(input[i].type == "checkbox") { 
       input[i].checked = false; 
      } 
     } 
    } 
   } 
  </script> 
 </body> 
</html> 
</body> 
</html> 

if(input[3].checked) { 

if(input[2].checked) {