Acces a une page suivante

<form method="post" action="">
Code de paiement   <input type="text" placeholder="Code de paiement" name="cod">
<input type="submit" value="Suivant">
</form>
<?php
if (empty($_POST['cod'])){
    echo "Saississez le code";
 }
     else{
  $user="root";
 $pwd="";
 $host="localhost";
 $bd="soutenance";
 $cnx=mysql_connect($host,$user,$pwd);
 mysql_select_db($bd,$cnx);
 $req=mysql_query("SELECT * FROM etudiant WHERE  CodEtudiant =' " .$_POST['cod']. " ' ") or die (mysql_error());
 $nbr = mysql_num_rows($req);
 if($nbr==1){
    echo "Ce code est deja attribué à un utilisateur";
    }
  else{
   $c=htmlentities(trim($_POST['cod']));
     $user="root";
      $pwd="";
     $host="localhost";
      $bd="soutenance";
        $cnx=mysql_connect($host,$user,$pwd);
       mysql_select_db($bd,$cnx);
        $req=mysql_query("SELECT * FROM codepaiement WHERE  codpaiement =' " .$_POST['cod']. " ' ") or die (mysql_error());
        $nbr=mysql_num_rows($req);
       if ($nbr==0){
                echo "Ce code n'est pas valide";}
       else {
             $c=htmlentities(trim($_POST['cod']));
                            $user="root";
                            $pwd="";
                            $host="localhost";
                            $bd="soutenance";
                            $cnx=mysql_connect($host,$user,$pwd);
                            mysql_select_db($bd,$cnx);
                            $req=("insert into etudiant (CodEtudiant) value ($c);") or die (mysql_error());
                            mysql_query($req);
                          mysql_close();
          
          }
          }
   }             
            
?>