Les session php

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-------------------interfacetransport.php-------------- 
<?php 
ob_start(); 
session_start(); 

if(isset($_SESSION['nom']) && isset($_SESSION['pass'])){ 
    header("Location: tr.php"); 
    exit(); 
    } 


?> 

-------------------------------------------- 
---------------------tr.php---------------- 
<?php 
ob_start(); 
session_start(); 
include "connexiontransport.php"; 

echo' <div class="lamine"></div>'; 
$query = mysql_query("select * from client order by n_expedition desc ") or die ("error mysql"); 
$combo = mysql_query("select DISTINCT destinateur from client order by destinateur asc ") or die ("error mysql"); 


$username= addslashes(stripcslashes($_POST['nom'])); 
$userpass= addslashes(stripcslashes($_POST['pass'])); 

if($username && $userpass){ 
    $finduser = mysql_query("select * from pass where nom ='".$username."' and pass='".$userpass."'") or die ("mysql error"); 
    if(mysql_num_rows($finduser) !=0){ 
        while($row = mysql_fetch_assoc($finduser)){ 
            $uname = stripcslashes($row['nom']); 
            $upass = stripcslashes($row['pass']); 
        } 
        if($username == $uname and $userpass == $upass){ 
            $_SESSION['nom'] = $uname; 
            $_SESSION['pass'] = $upass; 
            echo "<h1> welcome to ".$uname." your menu </h1> 
               
             
              "; 
               
          }else{ 
            die("<h2> not found users</h2>"); 
          } 
            }else{ 
                die("<h5>mot de pass incorrect <a href='interfacetransport.php'> login </a></h5>"); 
                   
            } 
         
    } 
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------------------------------------------------------- 
--------------------------logouttt.php---------------- 
<?php 

ob_start(); 
session_start(); 
include "connexiontransport.php"; 
if(!isset($_SESSION['nom']) && !isset($_SESSION['pass'])){ 
    header("location: interfacetransport.php"); 
    exit(); 
 }else{ 

   header("location: "); 
   session_destroy(); 
   exit(); 
    } 
    
ob_end_flush(); 
 mysql_close($connectdb); 
?>