process.php

<?php
session_start();

//establish connection
$mysqli = new mysqli('localhost', 'root', '', 'crud-2') or die(mysqli_error($mysqli));

// set variables for edit click
$id = 0;
$update = false;
$title = "";
$author = "";

//check if addBook button is clicked
if (isset($_POST['addBook'])) {
    $title = $_POST['title'];
    $author = $_POST['author'];

    // insert data into database
    $insert = "INSERT INTO data (title , author) VALUES ('$title', '$author')";
    $mysqli->query($insert) or die($mysqli->error);

    $_SESSION['message'] = "A list without books is like a body without a soul. <br> Your book has been added!";
    $_SESSION['msg_type'] = "success";

    // redirect to index
    header("location: index.php");
}

//check if delete button is clicked with get because an url is passed
if (isset($_GET['delete'])) {
    $id = $_GET['delete'];

    $delete = "DELETE FROM data WHERE id=$id";
    $mysqli->query($delete) or die($mysqli->error);

    $_SESSION['message'] = "Your book has been deleted! Hopefully that was not a mistake!";
    $_SESSION['msg_type'] = "danger";

    // redirect to index
    header("location: index.php");
}

// check if edit button is clicked
if (isset($_GET["edit"])){
    $id = $_GET["edit"];
    $update = true;
    $result = $mysqli->query("SELECT * FROM data WHERE id=$id") or die($mysqli->error());

    // check if record exists
    if (count(array($result))==1){
        $row = $result->fetch_array();
        $title = $row["title"];
        $author = $row["author"];
    }
}

//check if update button is clicked
if (isset($_POST["update"])){
    $id = $_POST["id"];
    $title = $_POST['title'];
    $author = $_POST['author'];

    $updateQuery = "UPDATE data SET title = '$title', author='$author' WHERE id=$id";
    $mysqli->query($updateQuery) or die($mysqli->error);

    $_SESSION['message'] = "Hooray, your book entry has been updated!";
    $_SESSION['msg_type'] = "warning";

    // redirect to index
    header("location: index.php");
}