<?php

/*
 * Assuming data structure:
 *
 * mathbro
 * -------
 * id | question | answer1 | answer2 | answer3 | answer4 | is_right
 * ----------------------------------------------------------------
 *
 */
/**
 * @param PDO     $conn
 * @param integer $question_id
 * @param integer $selected_answer
 *
 * @return bool
 */
function check_answer(PDO $conn, $question_id, $selected_answer) {
    //I'm doing it this way to not complicate things! Normally use prepared statements!
    $query = "SELECT `is_right` FROM `mathbro` WHERE `_id` = $question_id";

    //Preform query
    $stmt = $conn->query($query);
    //Fetch results
    $result = $stmt->fetchAll(PDO::FETCH_ASSOC);

    //Testing logic
    if ($result["is_right"] == $selected_answer) {
        return true;
    }
    return false;
}

/**
 * @param PDO        $conn
 * @param integer    $question_id
 *
 * @return array
 */
function fetch_question(PDO $conn, $question_id) {
    $query  = "SELECT * FROM `mathbro` WHERE `_id` = $question_id";
    $stmt   = $conn->query($query);
    $result = $stmt->fetchAll(PDO::FETCH_ASSOC);
    return $result;
}

//DB connection. Using PDO here, better than MySQLi imo.
try {
    $conn = new PDO("mysql:host=localhost;dbname=database", "root", "password");
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); //Set error reporting to exceptions. Helpful to find errors!
}
catch (PDOException $e) {
    die("Database connection failed! " . $e->getMessage());
}

//Check the current question
$current_question = 1;
if (isset($_POST["current_question"])) { //POST is set, question was answered.
    $current_question = $_POST["current_question"];

    //Check to see if answer is correct
    try {
        if (check_answer($conn, $current_question, $_POST["ans"])) { //Answer is correct
            $current_question++;
        }
    }
    catch (PDOException $e) {
        die("There was a problem! " . $e->getMessage());
    }
}
try {
    $question = fetch_question($conn, $current_question);

    ?>

<form action="math.php">

    <?php
    if ($current_question == $_POST["current_question"]) { //Variable was not increased, answer isn't correct
        echo "<p class=\"error\">Incorrect! Try again!</p>";
    }
    ?>

    <label><input type="radio" name="ans" value="1"><?php echo $question["answer1"]; ?></label>
    <label><input type="radio" name="ans" value="2"><?php echo $question["answer2"]; ?></label>
    <label><input type="radio" name="ans" value="3"><?php echo $question["answer3"]; ?></label>
    <label><input type="radio" name="ans" value="4"><?php echo $question["answer4"]; ?></label>

    <input name="current_question" type="hidden" value="<?php echo $current_question; ?>">

    <button type="submit">Next</button>
</form>
<?php
}
catch (PDOException $e) {
    die("There was a problem! " . $e->getMessage());
}
?>