Erreur de la mise à jour mysql php

Bonjour,

J'aimerai faire la mise à jour des quelques colonne des informations d'un membre inscrit dans ma base des données mais je rencontre plusieurs problème.

Voici les erreurs que je rencontre :

Notice: Undefined index: u in C:\wamp\www\internetservices\modifierAgent.php on line 6

Notice: Undefined index: u in C:\wamp\www\internetservices\modifierAgent.php on line 6
Erreur SQL !UPDATE cybernet.agent SET nom = "", prenom = "", privilege = "Administrateur", email = "", telephone = "", fonction_tech = "Sales" WHERE agent.0=
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 2

Toute mes sources

<?php
//connection au serveur:
$cnx = mysql_connect( "localhost", "root", "" ) ;
//sélection de la base de données:
$db = mysql_select_db( "mabase" ) ;

 if (isset($_POST['Modifier']) && $_POST['Modifier'] == 'Modifier') {

 		{
 	$sql = 'UPDATE cybernet.agent SET nom = "'.mysql_escape_string($_POST['nom']).'", prenom = "'.mysql_escape_string($_POST['prenom']).'", privilege = "'.mysql_escape_string($_POST['privilege']).'", email = "'.mysql_escape_string($_POST['email']).'", telephone = "'.mysql_escape_string($_POST['telephone']).'",
fonction_tech = "'.mysql_escape_string($_POST['fonction_tech']).'" WHERE agent.'.intval($_GET['u']).'='.$_SESSION['u'].'';
mysql_query($sql) or die('Erreur SQL !'.$sql.'<br />'.mysql_error());
 	echo '<div class="succes">Modifications faites avec succès</br></div>';
 		}
 }
 	else{
 		$erreur = 'Attention aux erreurs, remplissez correctement le formulaire pour modifier vos informations.';
 	}
?>
<div id="box_Modif_Agent">
<form action="" method="post" name="form1" id="form1">
          <table width="320" border="0" class="left">
 <?php
   if (isset($erreur)) {
   echo '<div id="info" class="worning">'.$erreur.'</div><!--error-->';
     } ?>
     <br/><br/>
     <h4>Modifier compte</h4>

            <tr>
              <td class="td">Nom</td>
              <td> <input type="text" name="nom" class="input_ajout_agent"  value="<?php if (isset($_POST['nom']))  echo htmlentities(trim($_POST['nom'])); ?>" ></td>
            </tr>
            <tr>

			   <tr>
              <td>Prenom</td>
              <td >
                  <input type="text" name="prenom"  class="input_ajout_agent" value="<?php if (isset($_POST['prenom'])) echo htmlentities(trim($_POST['prenom'])); ?>" ></td>
            </tr>

			   <tr>
              <td>Privilege</td>
              <td>
			  <select name="privilege" class="input_ajout_agent"  value="<?php if (isset($_POST['privilege'])) echo htmlentities(trim($_POST['privilege'])); ?>"  >
			  <option>Administrateur</option>
			  <option>Utilisateur</option>
			  </select>
			  </td>
            </tr>

             <tr>
              <td class="td">E-mail</td>
              <td> <input type="text" name="email" class="input_ajout_agent"  value="<?php if (isset($_POST['email'])) echo htmlentities(trim($_POST['email'])); ?>"></td>

			   	<tr>
              <td class="td">Téléphone</td>
                     <td> <input type="text" class="input_ajout_agent"  name="telephone" value="<?php if (isset($_POST['telephone'])) echo htmlentities(trim($_POST['telephone'])); ?>"></td>
            </tr>
              <tr>
              <td class="td">Fonction</td>

               <td>
			  <select name="fonction_tech" class="input_ajout_agent"  value="<?php if (isset($_POST['fonction_tech'])) echo htmlentities(trim($_POST['fonction_tech'])); ?>"  >
			  <option>Managere</option>
			  <option>Sales</option>
			  <option>Technicien</option>
			  </select>
			  </td>
            </tr>

            <tr>
            <td></td>
              <td><input type="submit" name="Modifier" class="submi" value="Modifier" /></td>
            </tr>
			</table>

</form></div>
</body>
</html>

Ma table agent

CREATE TABLE IF NOT EXISTS 'agent' (
  'id' int(11) NOT NULL AUTO_INCREMENT,
  'nom' varchar(100) COLLATE utf8_unicode_ci NOT NULL,
  'prenom' varchar(100) COLLATE utf8_unicode_ci NOT NULL,
  'password' varchar(100) COLLATE utf8_unicode_ci NOT NULL,
  'matricule' varchar(45) COLLATE utf8_unicode_ci NOT NULL,
  'privilege' varchar(50) COLLATE utf8_unicode_ci NOT NULL,
  'date' datetime NOT NULL,
  'email' varchar(100) COLLATE utf8_unicode_ci NOT NULL,
  'telephone' varchar(45) COLLATE utf8_unicode_ci NOT NULL,
  'fonction_tech' varchar(45) COLLATE utf8_unicode_ci NOT NULL,
  'birthday' date NOT NULL,
  'intervention' varchar(100) COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY ('id'),
  UNIQUE KEY 'id_agent_UNIQUE' ('id')
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=38 ;

Merci pour votre aide