My PDO query isn't work when parameter is null

Hi guys,
I'm trying to display users randomly along with data pull from the database. The code below used to work great with mysqli method. It's not working now:

if( logIn() ) {
    $id = $_GET['id'];
} else {
    $id = null;
}
$sql = "
    SELECT
        user.id, user.name, user.cat, user.privacy, user.banned, snapshot.uid, snapshot.img as img, snapshot.avt as avt
    FROM
        user
    LEFT JOIN
        (SELECT snapshot.uid, snapshot.img, snapshot.avt FROM snapshot) snapshot
    ON
        user.id = snapshot.uid AND snapshot.avt = '1'
    WHERE
        user.id != :id AND user.banned != :banned AND RAND()<(SELECT ((15/COUNT(*))*10) FROM user)
    GROUP BY
        user.id
    ORDER BY RAND()
    LIMIT 15
";
$banned = 1;
try {
    $stmt = $conn->prepare($sql);   
    $stmt->execute(array(':id' => $id, ':banned' => $banned));
    $users = array();
    $users = $stmt->fetchAll(PDO::FETCH_ASSOC);
    if (count($users) <> 0 ) {
        foreach ( $users AS $user ) {
            echo $user['name'];
        }
    } else {
        echo "No user!";
    }
} catch(PDOException $e) {
    echo $e->getMessage();
}

Code (markup):

Please help and thank you,

 

Why do you have this?

    AND RAND()<(SELECT ((15/COUNT(*))*10) FROM user)

Code (markup):

Anyways, are you getting an error? Could you try this?

    $stmt = $conn->prepare($sql);
    $stmt->bindValue('id',$id, PDO::PARAM_INT);
    $stmt->bindValue('banned',$banned, PDO::PARAM_INT);
    $stmt->execute();

Code (markup):

 

Thanks for help. It doesn't work. It looks pretty pattern anyway. I may consider using it.

Base on my research, it's the most efficient way (in term of fastest) to random large database. If I do not misunderstand, it pick up amount of data in range and ignore the others. It may not be very efficient in term of coverage, but I focus on speed.