Direct access Action Form

Talk about people, recently I started to study php and I am facing some problems, the most recent was as follows.

I have a login / registration form and we have the following code.

<form action="/includes/login/class_registro.php" method="post">
            <div class="input-container">
                <input type="text" id="reg_username" required="required" name="username" autocomplete="off" />
                <label for="reg_username">Usuario</label>
                <div class="bar"></div>
            </div>
            <div class="input-container">
                <input type="email" id="reg_email" required="required" name="email" autocomplete="off"/>
                <label for="reg_email">Email</label>
                <div class="bar"></div>
            </div>
            <div class="input-container">
                <input type="password" id="reg_pwd" required="required" name="password" autocomplete="off"/>
                <label for="reg_pwd">Senha</label>
                <div class="bar"></div>
            </div>
            <div class="input-container">
                <input type="password" id="reg_confirm_pwd" required="required" autocomplete="off"/>
                <label for="reg_confirm_pwd">Repita a senha</label>
                <div class="bar"></div>
            </div>
            <div class="button-container">
                <button name="btn_rg" value="enviar"><span>Concluir</span></button>
            </div>
        </form>

The problem is that if the user does www.mysite.com/includes/login/class_registro.php he can access the file normally, so I did the following:

//Verifica se o acesso foi realizado diretamente pela URL
$enviou = (isset($_POST["btn_rg"]) && !empty($_POST["btn_rg"])) ? true :          false;
if (!$enviou) {
header("Location: http://www.meusite.com.br/login");
}
//Verifica se o acesso foi realizado diretamente pela URL

But I was in doubt if there would be some way "better" to do this procedure, the same happens to www.mysite.com/includes/login/class_login.php