Whats the problem of this code? php: function users_online() {

Question

Whats the problem of this code? php: function users_online() {

if(isset($_GET['onlineusers'])) {

global $connection;

if(!$connection) {

session_start();

include("../includes/db.php");

$session = session_id();
$time = time();
$time_out_in_seconds = 30;
$time_out = $time - $time_out_in_seconds;

$query = "SELECT * FROM users_online WHERE session = '$session'";
$send_query = mysqli_query($connection, $query);
$count = mysqli_num_rows($send_query);

if($count == NULL) {

mysqli_query($connection, "INSERT INTO users_online(session, time) VALUES('$session','$time')");

} else {

mysqli_query($connection, "UPDATE users_online SET time = '$time' WHERE session = '$session'");

}

$users_online_query = mysqli_query($connection, "SELECT * FROM users_online WHERE time > '$time_out'");
$count_user = mysqli_num_rows($users_online_query);
echo $count_user;

}

} // get request isset()
}

users_online();

js:
function loadUsersOnline() {

$.get("functions.php?onlineusers=result", function(data){

$(".usersonline").text(data);

});

}

setInterval(function(){

loadUsersOnline();

},500);

html:
<li><a href="">Online Users: <span class="usersonline"></span></a></li>