Basic sanitization with filter_var()

<?php
// variables to represent different message generated depending on user input submission
$validation_error = ""; //the inner HTML of our error <span>.
$user_answer = "";  // assigned to the value attribute of the "answer" input element
$submission_response = "";

// Write Function here
if ($_SERVER["REQUEST_METHOD"] === "POST") {
  $user_answer = filter_var($_POST["answer"], FILTER_SANITIZE_NUMBER_INT); // inbuilt sanitization filter to sanitize integers. First argument - variable, second argument - takes an ID representing the type of filtering that should be performed.
  if ($user_answer != "-5"){
    $validation_error = "* Wrong answer. Try again.";
  } else {
    $submission_response = "Correct!";
  }
}
?>

<h2>Time for a math quiz!</h2>
<form method="post" action="">
<h4>Question 1:</h4>
<p>What is 6 - 11?</p>
<input type="text" name="answer" id="answer" value="<?= $user_answer;?>">
<br>
<span class="error" id="error"><?= $validation_error;?></span> <!--if wrong, message to print will be here -->
<br>
<input type="submit" value="Submit Your Answer">
</form>
<div>
  <p id="answer-display">Your answer was: <?= $user_answer;?></p>
  <p id="submission-response"><?= $submission_response;?></p> <!-- print message when it's correct answer-->
</div>