즉.

<?
$all_sum  = "";
if($ext10_01){
$all_sum +=  $ext10_01;
}

if($ext10_02){
$all_sum +=  $ext10_02;
}

if($ext10_03){
$all_sum +=  $ext10_03;
}

if($ext10_04){
$all_sum +=  $ext10_04;
}

if($ext10_05){
$all_sum +=  $ext10_05;
}

$rowsum = sql_fetch(" select sum($all_sum) as sum_value from $write_table ");

//생략


^^ 조금 무식하게 했나. ㅠ ;;

무식하게 하는게 때론..간단하게 처리가 ^^

$all_sum = "0";
for($i=1;$i<6;$i++)
{
$all_sum += ${ext10_0}.$i;
}

echo $all_sum;