Adding multiple checkbox values to multiple columns in a single row

[GlobalVillage]

hi - i read other threads but don't think i have found the answer (or maybe just don't know how to implement it...) for this. i have a mysql table with columns for an item size, amongst other things. the user fills in a form in which mulitple sizes may be selected from a series of checkboxes. i want to be able to insert the selected checkbox values in to the proper columns while leaving the other columns either null or if necessary i guess as a 0 value. the columns are tinyint format right now.

'One Size'

<input type="checkbox" name="itemSize[]" value="1" />

Small

<input type="checkbox" name="itemSize[]" value="2" />

Medium

<input type="checkbox" name="itemSize[]" value="3" />

Large

<input type="checkbox" name="itemSize[]" value="4" />

the database table has columns named oneSize, small, medium and large.

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HERE IS THE CODE I USED TO TRY TO INSERT A RECORD BASED ON AN ARTICLE IN FOUND IN THE FORUM:

$itemName = $_POST["itemName"];
$allsizes = $_POST['itemSize'];
var_dump($allsizes);
$itemSizing = "";
foreach ($allsizes as $size) {
$itemSizing .=$size . ", ";
}
$itemSizing = substr($allsizes, 0, -2);
$price = $_POST["price"];
$smallPrice = $_POST["smallPrice"];
$mediumPrice = $_POST["mediumPrice"];
$largePrice = $_POST["largePrice"];
$description = $_POST["description"];
$category_id = $_POST["categoryName"];

$SQL="INSERT INTO menuItems (itemName, oneSize, small, medium, large, price, smallPrice, mediumPrice, largePrice, description, category_id) VALUES ('$itemName', '$itemSizing', '$price', '$smallPrice', '$mediumPrice', '$largePrice', '$description', '$category_id')";

-----------------------------------

THIS IS THE ERROR MESSAGE I GOT WHEN ATTEMPTING TO INSERT A RECORD:

array(2) { [0]=> string(1) "2" [1]=> string(1) "4" } 
Warning: substr() expects parameter 1 to be string, array given in menuItems.php on line 162
INSERT INTO menuItems (itemName, oneSize, small, medium, large, price, smallPrice, mediumPrice, largePrice, description, category_id) VALUES ('Garden Salad', '', '', '3.99', '', '6.99', 'Fresh greens with lettuce, tomatoes and cucumbers. Served with your choice of dressing.', '3') : Error in query. Contact Site Admin

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i thought that the string being created was a series of the checkbox values separated out for insertion into the various columns - which is exactly what i want to happen - but clearly it didn't work. i am obviously not very familiar with arrays so i apologize if this is a simple question.

thanks very much for any help anyone can provide...