update data on mysql using ajax

I'm trying to send the id of an image when a user click on it using ajax. Then in the update.php file I'd like to update the id of the image. I'm new on ajax, so I'm not able to figure out my error.

$(document).ready(function(){
$('.show_people_image .love').on('click', function() {

     var id = $(this).attr('id');
     var idpage = "<?php echo $id ?>";
    var sVar1 = encodeURIComponent(id);
    var sVar2 = encodeURIComponent(idpage);
    var sVar3 = "<?php echo $_SESSION['aaa'] ?>";
    var sVar4 = "<?php echo $_SESSION['bbb'] ?>";

     if (sVar1 == sVar2) {
    $.ajax({
        type: "POST",
        url: "update.php",
        data: {lid:sVar1, ml:sVar3, mem:sVar4},
        success: function(data) {
            alert(data);
            },
        error: function () {
        alert('error');
        }  
     });    
     } 
});
});

The update.php file is:

if (isset($_POST['lid']) AND isset($_POST['ml']) AND isset($_POST['mem'])) {

include 'models/connexion_sql.php';


$loveid = $_POST['lid'];

if ($_POST['ml']) {
$str = $_POST['ml'].','.$loveid;
}
else {
$str = $loveid;
}


$sql = 'UPDATE users SET myl= :myl WHERE email = :email';
$req = $bdd->prepare($sql);
$req->bindParam(':email', $_POST['mem'], PDO::PARAM_STR);
$req->bindParam(':myl', $str, PDO::PARAM_STR);
$req->execute();

echo $str;

}  

If I put an echo in the upload.php file before the prepare($sql) the request is successful, otherwise the request fail.

via Chebli Mohamed