Something wrong with this code? [closed]

Hey been at this for a day can't fix the problem. This function updates user info in the db [MySQL] it keeps throwing the exception Couldn't update user

<?php
function update_user($user_id, $update_data) {
    global $db;
    $update = array();//array_walk($update_data, 'array_sanitize');

    foreach ($update_data as $field=>$data) {
        if ($field != 'allow_email') {
        $update[] = '`' . $field . '` = \'' . $data . '\'';
        } else {
            $update[] = '`' . $field . '` = ' . $data;
        }
    }
    $updateImploded = implode(', ', $update);
    var_dump($updateImploded);
    $stmt = $db->prepare("UPDATE `users` SET :updateImploded WHERE `user_id` = :user_id");
    try {
        $success = $stmt->execute(array(':updateImploded'=>$updateImploded, ':user_id'=> $user_id));
        var_dump($stmt);
    } catch (PDOException $e) {
        die ('Couldn\'t update user');
    }
    //mysql_query("UPDATE `users` SET" . implode(', ', $update) . " WHERE `user_id` = '$user_id'");
}

EDIT 2 After revising it to use php to prepare the statement

    foreach ($update_data as $field=>$data) {
        $update[] = '`' . $field . '` = \'' . $data . '\'';
    }
    $updateImploded = 'UPDATE `users` SET' . implode(', ', $update) . 'WHERE `user_id` =' .$user_id;
    $stmt = $db->prepare($updateImploded);
    try {
        $success = $stmt->execute();
        var_dump($stmt);
    } catch (PDOException $e) {
        die ('Cant update user');
    }
}