How to display previously assigned variable from past session - php

Question

I'm a new coder and I'm trying to write some code for a simple math game where you have to give the value of the second int in order to get to the sum.
I have it so the user submits their answer through a form and I want to have it so it checks the answer and if it is wrong it will display the correct answer. The issue I'm having is that after the user answers the question a new one is loaded and the code is checking the answer for the new question instead of the old one, displaying the answer to the new question.
For example, if the sum of int 1 and int 2 is 30 and the value of int 1 is 20, the value of int 2 must be 10 which is what the user would submit into the form.
I want to have it display "correct" or, if the user got it wrong, then "the correct answer is 10" but this is not the case because after the user answers the question it checks the answer for the new generated question and ends up displaying "the correct answer is 20" or whatever the new value of int 2 is. I do not know how to have it evaluate the answer for the old question as the variables are overwritten with the new question.
<?php
echo "<h2 id='heading'>php program to add two numbers...</h2><br />";
$val1 = rand(1, 100);
$val2 = rand(1, 100);
$sum = $val1 + $val2; /* Assignment operator */
echo "Result(SUM): $sum";
echo "<img src='math.jpg' alt='math' height='200' width='200'>";
echo "<h2>If val1 : $val1, what is val2</h2>";
echo "<form method = 'post'><input type='text' id='val2' name='val2' onChange='preventDefault();validateForm();'></form>";
if (strtolower($_SERVER['REQUEST_METHOD']) == 'post') {
echo ('The Value You Choose Was: ' . $_POST['val2'] . '<br/>');
$_SESSION['value2'] = $val2;
if ($_POST['val2'] == $val2 && isset($_POST['val2'])) {
echo 'You Answered Correctly';
} elseif(isset($_POST['val2'])) {
echo ('Wrong the correct answer was: ' . $_SESSION['value2']);
}
}
?>

Answer 1

The problem is that you are regenerating $val1 and $val2 again before checking their response.
Lets say, on the first page load $val1 = 1 and $val2 = 2.
The screens should say something akin to 1 + ? = 3.
The user (correctly) answers 2 and submits the form. PHP starts from square one every time you run the script - so the first thing it'll do is generate $val1 = 5 and $val2 = 3. This has changed the answer. It is no longer correct.
So basically you need to save $val1 and $sum in your form (possibly as hidden fields) so that you can reference them when checking the answers. When the REQUEST_METHOD is POST - read from those instead of generating new ones.

Answer 2

As you might already imagine, your program needs to remember the values of the random numbers between the two requests. To achieve this, you can use session varibales. A session variable can be accessed during multiple requests, until the session expires.
http://php.net/manual/en/session.examples.basic.php
You already assing the result of your game to a session variable:
$_SESSION['value2'] = $val2;
But thats not enough:
start the session with session_start();
in your if comparison, use the session variable (so you compare to the value of the previous request) instead of the newly created value
assign the session variable after your if comparison (otherwise the session variable would have the new random nubmer instead of the previous one)
Final code (untested):
<?php
session_start();
echo "<h2 id='heading'>php program to add two numbers...</h2><br />";
$val1 = rand(1, 100);
$val2 = rand(1, 100);
$sum = $val1 + $val2; /* Assignment operator */
echo "Result(SUM): $sum";
echo "<img src='math.jpg' alt='math' height='200' width='200'>";
echo "<h2>If val1 : $val1, what is val2</h2>";
echo "<form method = 'post'><input type='text' id='val2' name='val2' onChange='preventDefault();validateForm();'></form>";
if (strtolower($_SERVER['REQUEST_METHOD']) == 'post') {
echo ('The Value You Choose Was: ' . $_POST['val2'] . '<br/>');
if (isset($_POST['val2']) && $_POST['val2'] == $_SESSION['value2']) {
echo 'You Answered Correctly';
} elseif(isset($_POST['val2'])) {
echo ('Wrong the correct answer was: ' . $_SESSION['value2']);
}
}
$_SESSION['value2'] = $val2;