form.php

<?php
$period = isset($_POST['period']) ? trim($_POST['period']) : '';

echo <<<EOF
<form method="post" action="{$_SERVER['REQUEST_URI']}">
  <div>
    <label for="period">Periodo:</label>
    <input type="text" name="period" id="period" value="{$period}">
  </div>

  <div>
    <button type="submit">Invia</button>
  </div>
</form>
EOF;

if ('POST' === $_SERVER['REQUEST_METHOD']) {
  if ('' === $period) {
    echo '<p>Periodo non inserito.</p>';
  } else {
    try {
      $pdo = new \PDO('mysql:host=localhost;dbname=database', 'user', 'pass');
    } catch (\PDOException $e) {
      die($e->getMessage());
    }

    $stm = $pdo->prepare('SELECT * FROM elements WHERE period = ?');

    try {
      $stm->execute($period);
    } catch (\PDOException $e) {
      die($e->getMessage());
    }

    echo <<<EOF
      <table>
        <thead>
          <tr>
            <th>Numero</th>
            <th>Nome</th>
            <th>Gruppo</th>
            <th>Periodo</th>
          </tr>
        </thead>

        <tbody>
EOF;

    while ($row = $stm->fetchAssoc()) {
      echo <<<EOF
        <tr>
          <td>{$row['number']}</td>
          <td>{$row['name']}</td>
          <td>{$row['group']}</td>
          <td>{$row['period']}</td>
        </tr>
EOF;
    }

    echo <<<EOF
        </tbody>
      </table>
EOF;
  }
}