All Experts Answer

<?php
//index.php

$correct_answer = 'Mark';

if($_SERVER['REQUEST_METHOD'] == 'POST') {
  if(!isset($_POST['dad') || $_POST['dad'] !== $correct_answer)
  {
    $error = '<p>You answered your father\'s name wrong. Please try again.';
  }
  else
  {
    header("Location:q2.php");
  }
}

if(!empty($error))
  echo $error;
?>

<form action="index.php" method="post">
    <label>Enter your father's name:</label>
    <input autofocus required autocomplete="off" type="text" name="dad" placeholder="Bill">
    <input type="submit" value="Press me when you think you are right" />
</form>