Ajax in PHP

<script src="https://code.jquery.com/jquery-3.4.1.min.js" type="text/javascript"></script>
<script type="text/javascript">
$(document).ready(function(){
    $('#button1').click(function(){
        $.ajax({
            type:'POST',
            url:'ajax1.php',
            success:function(result){
                $('#result').html(result);
            }
        });
    });

    $('#button2').click(function(){
        $.ajax({
            type:'POST',
            data:{a:1,b:2},
            url:'ajax2.php',
            success:function(result){
                $('#result').html(result);
            }
        });
    });

    $('#button3').click(function(){
        $.ajax({
            type:'POST',
            url:'ajax3.php',
            success:function(result){
                var product = $.parseJSON(result);
                var s = 'Id: ' + product.id;
                s += '<br>Name: ' + product.name;
                s += '<br>Price: ' + product.price;
                $('#result').html(s);
            }
        });
    });

    $('#button4').click(function(){
        $.ajax({
            type:'POST',
            url:'ajax4.php',
            success:function(result){
                var listProducts = $.parseJSON(result);
                var s = '';
                for(var i=0; i<listProducts.length; i++){
                    s += '<br>Id: ' + listProducts[i].id;
                    s += '<br>Name: ' + listProducts[i].name;
                    s += '<br>Price: ' + listProducts[i].price;
                    s += '<br>===============';
                }
                $('#result').html(s);
            }
        });
    });

});
</script>
<form>
<input type="button" value="Hello" id="button1">
<input type="button" value="Sum" id="button2">
<input type="button" value="Product Info" id="button3">
<input type="button" value="Product List" id="button4">
<br>
<div id="result"></div>
</form>
            

<?php
class Product{
    var $id;
    var $name;
    var $price;
}
?>

<?php
echo 'Hello World';
?>

<?php
echo $_POST['a'] + $_POST['b'];
?>

<?php
require 'product.php';
$product = new Product();
$product->id = 'p1';
$product->name = 'Product 1';
$product->price = 100;
echo json_encode($product);
?>

<?php
require 'product.php';
$listProducts = array();

$product1 = new Product();
$product1->id = 'p1';
$product1->name = 'Product 1';
$product1->price = 100;
array_push($listProducts, $product1);

$product2 = new Product();
$product2->id = 'p2';
$product2->name = 'Product 2';
$product2->price = 200;
array_push($listProducts, $product2);

$product3 = new Product();
$product3->id = 'p3';
$product3->name = 'Product 3';
$product3->price = 300;
array_push($listProducts, $product3);

echo json_encode($listProducts);
?>