Simplify the very long if statement in PHP

$gateway_site_id = $_POST['gateway_site_id'];
$site_location = $_POST['site_location'];
$city = $_POST['city'];
$contact = $_POST['gateway_site_id'];
$date_installed = $_POST['date_installed'];
$care_of = $_POST['care_of']; 
$notes = $_POST['notes'];
$gateway_username = $_POST['gateway_username'];



if(empty($gateway_username)){
$gateway_username="N/A";
}if(empty($notes)){
$notes="N/A";
}if(empty($care_of)){
$care_of="N/A";
}if(empty($date_installed)){
$date_installed="N/A";
}if(empty($city)){
$city="N/A";
}if(empty($site_location)){
$site_location="N/A";
}if(empty($gateway_site_id)){
$gateway_site_id="N/A";
}

When I submit my form, I just want to set empty fields with N/A. My code is working but I think its too long, is there a way to simplify it?