How to deal with inconsistent PHP warnings (especially in context of local hosting vs online hosting)?

Question

I know my code is shabby, but I don't think it's really that at fault for it here?

Managed to work out how to redirect users to different pages depending on their level as given through a login/register form, and it worked okay locally on my computer. No errors or bugs detected, and by manual testing it seemed okay.

I then uploaded the entire login/register program on a free hosting site (000webhost) with essentially identical code, yet the outcomes are very different for my validation process: post-login screen, locally hosted; post-login screen, on 000webhost.

Specifically, the errors are:

Warning: Undefined array key "level" in /storage/ssd1/382/21909382/public_html/validation.php on line 15

Warning: Undefined array key "admin" in /storage/ssd1/382/21909382/public_html/validation.php on line 27

Warning: Cannot modify header information - headers already sent by (output started at /storage/ssd1/382/21909382/public_html/validation.php:15) in /storage/ssd1/382/21909382/public_html/validation.php on line 28

The lines in question are:

Line 15: $level = $_POST['level'];

Line 27: $_SESSION["admin"];

Line 28: header('location:Admin Page.php');

Here is the complete file where the errors appear:

<?php

session_start();

$con = mysqli_connect('localhost','id21909382_biyo','!Biyo123');

mysqli_select_db($con, 'id21909382_access_levels');

if(isset($_POST['submit'])){

$name = $_POST['user'];
$pass = $_POST['password'];
$level = $_POST['level'];

echo $value ?? '';

$select = " select * from usertable where name = '$name' && password = '$pass'";

$result = mysqli_query($con, $select);

if (mysqli_num_rows($result) > 0){ 
    $row = mysqli_fetch_array($result);

    if($row['level'] == 'admin'){
        $_SESSION["admin"];
        header('location:Admin Page.php');

    }elseif($row['level'] == 'user'){
        $_SESSION["user"];
        header('location:User Page.php');
        
    }

}else{
    header('location:index.php');
}

}

?>

It's already strange enough that the two have vastly different outcomes despite being the same (the only thing that varies between them is $con = mysqli_connect('localhost','id21909382_biyo','!Biyo123');) due to 000webhost having different credentials for phpmyadmin. Even worse, all of the error-lines have similar ones that are error-free: line 27's $_SESSION["admin"]; is similar to $_SESSION["user"]; just a few lines after, for example. Much of the code is also copy-pasted from the registration file within my program, yet no problems there.

What exactly did I do wrong that is causing this discrepancy?

Answer 1

The problem eventually resolved itself. Randomly tried logging in again and it spontaneously worked.