4-Part Password Generator

<?php
  $alphabetUpper = array("A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z");
  $alphabetLower = array();
  foreach ($alphabetUpper as $letter) {
    array_push($alphabetLower, strtolower($letter));
  }
  $numbers = range(0, 9);
  $specialChars = array("!", "@", "#", "$", "%", "^", "&", "*", "_", "|", "/", "\\", ".");

  $characterSet = array_merge($alphabetUpper, $alphabetLower, $numbers, $specialChars);

  function generateRandomPassword() {
    global $characterSet;
    $count = (count($characterSet) - 1);
    $segment1 = ($characterSet[rand(0, $count)] . $characterSet[rand(0, $count)] . $characterSet[rand(0, $count)]);
    $segment2 = ($characterSet[rand(0, $count)] . $characterSet[rand(0, $count)] . $characterSet[rand(0, $count)]);
    $segment3 = ($characterSet[rand(0, $count)] . $characterSet[rand(0, $count)] . $characterSet[rand(0, $count)]);
    $segment4 = ($characterSet[rand(0, $count)] . $characterSet[rand(0, $count)] . $characterSet[rand(0, $count)]);
    return ($segment1 . "-" . $segment2 . "-" . $segment3 . "-" . $segment4);
  }
?>

<!DOCTYPE html>
<html lang="en">
  <head>
    <!-- Created by Thomas Jones of TomTec Solutions (07/08/16) -->
    <title>4-Part Password Generator</title>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.2/jquery.min.js"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
  </head>
  <body>
    <div class="container">
      <div class="jumbotron" style="text-align: center;">
        <h1><strong>4-Part Password Generator</strong></h1>
        <p>Your generated 4-Part Password:</p>
        <pre><?php echo generateRandomPassword(); ?></pre>
        <a href="passgen.php">Re-Generate</a>
      </div>
    </div>
  </body>
</html>