Php mysqli_fetch_array is no working - php

Question

I tried to create a register login feature in Android app. I tried to send data from my app to database by using this Php file. However, I received br/ error when I clicked register button on my app. Look like mysqli_fetch_array is not working. Anyone know how to solve this problem? Thanks!
<?php
if($_SERVER['REQUEST_METHOD']=='POST'){
$name = $_POST['name'];
$username = $_POST['username'];
$password = $_POST['password'];
$email = $_POST['email'];
if($name == '' || $username == '' || $password == '' || $email == ''){
echo 'please fill all values';
}else{
require_once('dbConnect.php');
$sql = "SELECT * FROM users WHERE username='$username' OR email='$email'";
$result=mysqli_query($con,$sql);
$check = mysqli_fetch_array($result,MYSQLI_BOTH);
if(isset($check)){
echo 'username or email already exist';
}else{
$sql = "INSERT INTO users (name,username,password,email) VALUES('$name','$username','$password','$email')";
if(mysqli_query($con,$sql)){
echo 'successfully registered';
}else{
echo 'oops! Please try again!';
}
}
mysqli_close($con);
}
} else{
echo 'error';
}
dbConnect.php
<?php
define('HOST','localhost');
define('USER','username');
define('PASS','password');
define('DB','database');
$con = mysqli_connect(HOST,USER,PASS,DB) or die('Unable to Connect');

Answer 1

I think you should remove isset check. It'll always return true no matter what. Do count check instead.
if(count($check)){
echo 'username or email already exist';
}else{
$sql = "INSERT INTO users (name,username,password,email) VALUES('$name','$username','$password','$email')";
if(mysqli_query($con,$sql)){
echo 'successfully registered';
}else{
echo 'oops! Please try again!';
}
}