将选择插入到mysql db中

I have my proccess.php

//HERE...code to ckeck if is set or not

//If is set

$h1='';
$d1='';
$a1='';
$h2='';
$d2='';
$a2='';

if (isset($_POST['submit_x'])) {

if (isset($_POST['h1'])) {
$h1 = $_POST['h1'];

if ($h1 == 'h1') {
$h1 = '1';
}
}

//...etc

//and insert into db

$query="INSERT INTO eur_drb (h1, d1, a1, etc)VALUES($h1, $d1, $a1, $etc)"

Thing is that user can select only one of h1, d1, a1.

How do I insert into my db the value only of the selected one? e.g if user selects h1 to insert into database:

$query="INSERT INTO eur_drb (select1, etc)VALUES($h1, $etc)"