A checkbox group creates an array of the values that where selected.
The array needs to be converted to a comma separated list before is can be inserted to the database.
add the following code at line 1 of your page:
<?php
if($_SERVER["REQUEST_METHOD"] == "POST") {
foreach($_POST as $key => $val) {
if(is_array($_POST[$key])) $_POST[$key] = implode(", ", $_POST[$key]);
}
}
?>
this uses the implose function to convert the array to a comma separated list.