Problem with PHP checkboxes

Hi, everybody !

I'll try to explain as clear as possible

Here is what I have:
2 MySQL tables -
1st - list of items available for order (about 300 of items)
2nd - list of items that one particular client orders (somewhere
between 50 to 120 items).

One of my forms shows to the client a list of items available for his
order, also it has an option to add items to that list. - That where I
having a problem.
What I want to do is to display list of all items available for
everybody, but with items that client already have being checked and
disabled so he (client) can check additional items and submit changes
later.

The part where " ... items that client already have being checked and
disabled ... " apparently being a problem.

Here is a part of code that I'm using:
<?
....

Querying part and beginning of HTML goes here

....

while ($row_rs_items = mysql_fetch_assoc($rs_items)) {

$row_rs_list = mysql_fetch_assoc($rs_list);

if (array_search($row_rs_items['item_num'], $row_rs_list) == NULL)
{
$flag = "";
} else {
$flag = " checked disabled";
}

?>

<tr bgcolor="<? echo $bgcolor; ?>">
<td><? echo $i; ?></td>
<td><? echo $row_rs_items['item_num']; ?></td>
<td align="left">&nbsp;&nbsp;&nbsp;<? echo
$row_rs_items['description']; ?></td>
<td><? echo $row_rs_items['vendor_name']; ?></td>
<td><? echo $row_rs_items['unit']; ?></td>
<td><div align="center">
<input type="checkbox" name="<? echo
$row_rs_items['item_num']; ?>" value="checkbox"<? echo $flag; ?>>

....

Any ideas ?
Any suggestions are deeply appreciated!

Thanks.