Simple form validation

<?php
// Define checkWord() here:
  function checkWord($input, $letter){
      if ($_SERVER["REQUEST_METHOD"] === "POST" && strtolower($input[0]) != $letter){  //also to check same letter despite whether upper or lower case
        return "* This word must start with the letter $letter!";
      }
      else {
        return "";
      }
  }

?>

<h1>Time to Practice our ABCs</h1>
<form method="post" action="">
    Enter a word that starts with the letter "a":
    <br>
    <!-- Assign the value attribute of each input tag to the value submitted by a user. User should be able to see what they entered in input field once they submit their form.  -->
    <input type="text" name="a-word" id="a-word" value= "<?= $_POST["a-word"]; ?>">
    <br>
    <!-- Invoke the function here to pass user's input into function for checking -->
      <p class="error" id="a-error"><?= checkWord($_POST["a-word"], "a"); ?></p>
    <br>
    Enter a word that starts with the letter "b":
    <br>
    <input type="text" id="b-word" name="b-word" value= "<?= $_POST["b-word"]; ?>">
    <br>
      <p class="error" id="b-error"><?= checkWord($_POST["b-word"], "b"); ?></p>
    <br>
    Enter a word that starts with the letter "c":
    <br>
    <input type="text" id="c-word" name="c-word" value= "<?= $_POST["c-word"]; ?>">
    <br>
      <p class="error" id="c-error"><?= checkWord($_POST["c-word"], "c"); ?></p>
    <br>
    <input type="submit" value="Submit Words">
</form>
<div>
    <h3>"a" is for: <?= $_POST["a-word"];?><h3> <!-- What user entered can be seen here -->
    <h3>"b" is for: <?= $_POST["b-word"];?><h3>
    <h3>"c" is for: <?= $_POST["c-word"];?><h3>
<div>