Example code for PHP class

simple-questions.php

<?php
    // DATA
    $questions=[];
    $questions[] = [
        "q" => "1+2",
        "ans" => "3"
    ];
    $questions[] = [
        "q" => "3x5",
        "ans" => "15"
    ];
    $questions[] = [
        "q" => "6/2",
        "ans" => "3"
    ];
    $questions[] = [
        "q" => "3-5",
        "ans" => "-2"
    ];

    // LOGIC
    $currentNum = $_POST["num"] ?? 0;
    $q = $questions[$currentNum];


    if (isset($_POST["ans"])) {
        if ($_POST["ans"] == $q["ans"]) {
            $currentNum += 1;
            $q = $questions[$currentNum];
      
        }
    }
   
?>
<form action="" method="POST">
    <h1><?= $q["q"] ?></h1>
    Your answer:<br>
    <input type="text" name="ans" autofocus>
    <input type="hidden" name="num" value="<?= $currentNum ?>">
    <input type="submit">
</form>