// Carrega mac do banco
$sqlMac = "SELECT idmacmiguel, nome FROM macmiguel ORDER BY nome";
$stmtMac = $conn->query($sqlMac);
$placasmac = $stmtMac->fetchAll(PDO::FETCH_ASSOC);


// Seleciona mac do banco
 <label>Mac:</label>
    <select name="idmacmiguel" required>
        <option value="">Selecione...</option>
        <?php foreach ($placasmac as $cat): ?>
            <option value="<?= $cat['idmacmiguel'] ?>">
                <?= htmlspecialchars($cat['idmacmiguel']) ?>
            </option>
        <?php endforeach; ?>
    </select>