<form action="director" method="post">
<input name="name" placeholder="Ваше имя" type="text" value="">
<input name="phone" placeholder="Ваш номер телефона" type="text" value="">
<textarea name="msg" placeholder="Текст сообщения" value=""></textarea>
<input id="lastname" name="lastname" type="text">
<input class="sendzakaz" type="submit" value="Заказать">
</form>

<?php
function checkField(&$fields) {
    if(!empty($fields['lastname'])) {
        return false;
    } else {
    return true;
    }
}