PHP - Only send form if all these conditions are matched (true)

So I have these inputs from my form firstname, lastname, username, email, password.

But I only want to complete the registration form and save it into my DB when all of these contain any info given by the user. I have this right now but it only shows an error if the password and/or password confirmation field do not match.

if( !empty ($_POST)){

    if($_POST['password'] == $_POST['password_confirmation']) {
       $user = new User();
       $user->setFirstName($_POST['firstname']);
       $user->setLastName($_POST['lastname']);
       $user->setUsername($_POST['username']);
       $user->setEmail($_POST['email']);
       $user->setPassword($_POST['password']);

    if($user->register()){
        $user->login();
    }

    else{
    $error_confirmation = true;
    // This gives error in the front end if both of the PW do no match each other
   }
}

I guess I am supposed to do something like this?

if( !empty ($_POST['firstname']) && !empty ($_POST['lastname']) && !empty ($_POST['username']) && !empty ($_POST['email']) && !empty ($_POST['password'])){