php - Warning: Undefined array key appear in multiple file ← (PHP, MySQL)

I'm new to programming and now I'm working on a project, this project already have prototype that is made by senior student, but now there's have multiple error in php file, and I can't find a way to contact him, I google a lot and tried many way to fix it but doesn't work, this is really driving me crazy.

here some example:

Warning: Undefined array key "id" in C:\xampp\htdocs\warmhouse\aeadmin.php on line 9

 <?php
     $result=All("select * from admin where id='1".$_GET["id"]."'");
     foreach($result as $row)
     {   
     ?>

and this one:

C:\xampp\htdocs\warmhouse\aeditu.php on line 5
" enctype="multipart/form-data">
Warning: Undefined array key "id" in C:\xampp\htdocs\warmhouse\aeditu.php on line 8

<?php 

require_once('sql.php'); ?>
<h1>ChangeMember</h1><br>
<form id="form" method="post" action="api.php?do=editu&id=<?=$_GET["id"]?>" enctype="multipart/form-data">

<?php
    $result=All("select *from user where u_id='".$_GET["id"]."'");
      foreach($result as $row)
    {   
    ?>  

basically it's seems like every file that have $_GET or $_POST something like this will have error, if someone need more information, just ask, I've been trying to solve this by myself several weeks, thank you.

I'm on PC and using xampp and phpmyadmin

Solution:

The error explains it perfectly. There's no id key in the . If you visit yoursite.com/aeadmin.php it will throw an error because you aren't supplying the id parameter, but if you visit yoursite.com/aeadmin.php?id=123 it will run fine.

if (!isset($_GET["id"])) {
    die ("Missing ID parameter");
}

As a tangent, it appears you are wide open to SQL Injections and should use parameterized prepared statements instead of manually building your queries. See PDO or MySQLi prepared statements.