php - The user ID is not defined -

ok i'm trying list users in mysql , that's working when add profile.php?id= else code user id not defined. how set wont user id not defined. here code:

<?php include('config/db.php'); ?> <?php $host="localhost"; // host name  $username="root"; // mysql username  $password="password"; // mysql password  $db_name="login"; // database name  $tbl_name="users"; // table name  mysql_connect("$host", "$username", "$password")or die("cannot connect");  mysql_select_db("$db_name")or die("cannot select db"); //we check if members id defined if(isset($_get['user_id'])) { $id = intval($_get['user_id']); //we check if user exists $dn = mysql_query('select user_name, user_email users       user_id="'.$id.'"'); if(mysql_num_rows($dn)>0) {     $dnn = mysql_fetch_array($dn);     //we display user datas ?> profile of "<?php echo htmlentities($dnn['user_name']); ?>" : <table style="width:500px;"> <tr>     <td><?php if($dnn['avatar']!='') { echo '<img src="'.htmlentities($dnn['avatar'], ent_quotes, 'utf-8').'" alt="avatar"     style="max-width:100px;max-height:100px;" />'; } else { echo 'this user dont have avatar.'; } ?></td>     <td class="left"><h1><?php echo htmlentities($dnn['user_name'], ent_quotes, 'utf-  8'); ?></h1>     email: <?php echo htmlentities($dnn['user_email'], ent_quotes, 'utf-8'); ?><br />     user joined website on <?php echo date('y/m/d',$dnn['signup_date']); ?>  </td> </tr> </table> <?php } else {     echo 'this user dont exists.'; } } else { echo 'the user id not defined.'; } ?> users.php     <?php //we ids, usernames , emails of members $req = mysql_query('select `user_id`, `user_name`, `user_email`, `user_sign_up_date`  `users`'); while($dnn = mysql_fetch_array($req)) { ?> <tr>     <td class="left"><?php echo $dnn['user_id']; ?></td>     <td class="left"><a href="profile.php?<?php echo $dnn['user_id']; ?>"><?php echo  ($dnn['user_name']); ?></a></td>     <td class="left"><?php echo($dnn['user_email']); ?></td> </tr> <?php } ?> 

you query returning false on following line

$dn = mysql_query('select user_name, user_email users user_id="'.$id.'"'); 

you need find out why, should not attempt untill returns true:

$sql = sprintf("select user_name, user_email users user_id = %s",                 mysql_real_escape_string($id)); $dn = mysql_query($sql);  if(!$dn){   echo "mysql_query() failed";   exit(); }