Php - Code comparing value of database to the value send by android app

<?php
$connect = mysqli_connect("localhost", "db", "pass", "host");
$name = $_POST["name"];
$time = $_POST["time"];
$id = $_POST["id"];
function registerUser() {
global $connect, $name, $time, $id;
 $statement = mysqli_prepare($connect, "INSERT INTO leaderboard (name, time, id) VALUES (?, ?, ?)");
    mysqli_stmt_bind_param($statement, "sis", $name, $time, $id);
    mysqli_stmt_execute($statement);
    mysqli_stmt_close($statement);     
}
function nameAvailable() {
    global $connect, $id;
    $statement = mysqli_prepare($connect, "SELECT * FROM leaderboard WHERE id = ?"); 
    mysqli_stmt_bind_param($statement, "s", $id);
    mysqli_stmt_execute($statement);
    mysqli_stmt_store_result($statement);
    $count = mysqli_stmt_num_rows($statement);
    mysqli_stmt_close($statement); 
    if ($count < 1){
        return true; 
    }else {           
        return false; 
    }
}
$response = array();
$response["success"] = false;  
if (nameAvailable()){
    registerUser();
    $response["success"] = true;  
      } else {
        $sql = "SELECT time from leaderboard WHERE id = $id;";
$res = mysqli_fetch_assoc(mysqli_query($connect, $sql));
if($time < $res['time']){
        $sql = "UPDATE leaderboard SET time = $time where id=$id";
        $res = mysqli_query($connect, $sql);
        if(!$res){
            die("Query not executed " . mysqli_error($connect));
             $response["success"] = false;
            } else {
                    $response["success"] = true;
        }
    }
      }
    echo json_encode($response);
?>