PHP MYSQL从多个列中检索最小值

PHP非常新，所以请耐心等待。

我有一张有好几列的表id | onezw | twozw | threezw | fourzw | fivezw | sixzw | minzw

每列都有一个输入的数字，然后我需要"minzw"列来显示6列中的最小值。我有以下代码，但似乎无法解决问题。请有人把我推向正确的方向。

<?php
$host="localhost"; // Host name
$username="xxx"; // Mysql username
$password="xxx"; // Mysql password
$db_name="xxxx"; // Database name
$tbl_name="ZW"; // Table name

// Connect to server and select database.
mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");
// get value of id that sent from address bar
$id=$_GET['id'];
// Retrieve data from database
$sql="SELECT * FROM $tbl_name WHERE id='$id'";
$result=mysql_query($sql);
$minzwarray = array("onezw" , "twozw" , "threezw" , "fourzw" , "fivezw" , "sixzw");
$minzw = min($minzwarray);
$rows=mysql_fetch_array($result);
echo $minzw['minzw'];

mysql_close();
?>