Empêche l'affichage d'une page PHP

 echo $_SESSION['rang'];

$_SESSION['username']=$username;
$_SESSION['rang']=$rang;

<?php session_start(); ?>   

<form method="post" action="index.php" style="margin:0">   
               <div class="modal-body">   
               <fieldset><?php   

if(isset($_POST['submit']))   
{   
$username = htmlspecialchars(trim($_POST['username']));   
$password = htmlspecialchars(trim($_POST['password']));   

 if($username&&$password)   
 {   
 $password=md5($password);   
 $connect = mysql_connect('localhost', 'nomutilisateur', 'motdepasse');   
 mysql_select_db('nombdd');   
    
 $log=mysql_query("SELECT * FROM users WHERE username='$username' and password='$password'");   
 $rows=mysql_num_rows($log);   
 if($rows==1)   
 {   
    
 $_SESSION['username']=$username;   
 echo '<script language="Javascript">document.location.replace("accueil.php");</script>';   

    
 }else echo "<div class='alert-message error'><a class='close' href='#' onClick='$(this).parent().slideUp();'>x</a><p>   
 <strong>Attention !</strong> Nom d'utilisateur ou mot de passe incorrect.</p></div>";   
    
 }else echo "<div class='alert-message error'><a class='close' href='#' onClick='$(this).parent().slideUp();'>x</a><p>   
 <strong>Attention !</strong> Veuillez saisir tous les champs.</p></div>";   
}   
?>   
                  <div class="clearfix">   
                  <label>Identifiant</label>   
                  <div class="input">   
                    <input name="username" type="text" placeholder="Identifiant"  style="height:28px" value="<?php if (isset($_POST['username'])) echo htmlentities(trim($_POST['username'])); ?>"  required/>   
                  </div>   
                 </div>   
                 <div class="clearfix">   
                  <label>Mot de passe</label>   
                  <div class="input">   
                    <input type="password" name="password" placeholder="Mot de passe" style="height:28px" value="<?php if (isset($_POST['password'])) echo htmlentities(trim($_POST['password'])); ?>" required>   
                  </div>   
                 </div>   
                 </fieldset>   
               </div>   
              <div class="modal-footer">   
                <input type="submit" name="submit" value="Connexion" class="btn primary">   
                   
              </div>   
              </form> 

<?php session_start(); 
include("require/connect.php"); 

echo "$_SESSION['rang']" ; 
?>

<?php session_start();
include("connect.php");

if($_SESSION['username'] != $username){
	echo '<script language="Javascript">document.location.replace("index.php");</script>';
}
?>
  

 <?php session_start();
include("connect.php");

if($_SESSION['rang'] != $rang){
	echo '<script language="Javascript">document.location.replace("index.php");</script>';
}
?>

		//On selectionne les données
		$index = mysql_query("SELECT ******");
		//si pas de résultat
		if(mysql_num_rows($index) == 0)
		{
			echo '<div id="erreur">Aucunes données ne correspond à votre saisie!</div>';
		}
		else{		
			while($result = mysql_fetch_array($index)){
					//on créer les session
					$_SESSION['username']=$username;
					$_SESSION['rang']=$rang;
				}
			}

Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in /home/locasudf/public_html/admin/require/essais.php on line 10 
Aucunes données ne correspond à votre saisie!