Om du vill behålla det som en SQL-sats kan du:
$sql = "
SELECT DISTINCT 'make' as descr,make as val FROM items
UNION
SELECT DISTINCT 'model' as descr,model as val FROM items
UNION
SELECT DISTINCT 'year' as descr,year as val FROM items
UNION
SELECT DISTINCT 'month' as descr,month as val FROM items
UNION
SELECT DISTINCT 'day' as descr,day as val FROM items
UNION
SELECT DISTINCT 'hour' as descr,hour as val FROM items";
$result = @mysql_query($sql, $con) or die(mysql_error());
while($row = mysql_fetch_array($result)) {
$make_array[$row['descr']][]=$row['val'];
}