functions.php

<?php
/* GET functions */

$id = '';
$fname = '';
$lname = '';
$username = '';
$email = '';
$update = false;

$_SESSION['message'] = '';
$_SESSION['toggle'] = '';

// Delete function
if (isset($_GET['delete'])) {
   $id = $_GET['delete'];
   $sqlDelete = "DELETE FROM users WHERE id=$id";
   $deleted = mysqli_query($connection, $sqlDelete) or die(mysqli_error($connection));

   if ($deleted) {
      $_SESSION['message'] = 'User deleted successfully';
      $_SESSION['toggle'] = 'warning';      
      header('location:index.php');
   }
}

// Edit function
if (isset($_GET['edit'])) {
   
   $id = $_GET['edit'];

   $sqlFetch = "SELECT * FROM users WHERE id=$id LIMIT 1";
   $result = mysqli_query($connection, $sqlFetch);

   if (mysqli_num_rows($result) == 1) {

      $update = true;

      $row = mysqli_fetch_array($result);
      
      $id = $row['id'];
      $fname = $row['firstname'];
      $lname = $row['lastname'];
      $username = $row['username'];
      $email = $row['email'];
   }
}