In order for the page to work like this you must ensure that you have the array of values posting to a page to begin with. Do you have this part worked out? This is the first thing we were working on.
Once you have the list posting an array of values you would then store the imploded array into your db.
On the update page you will have a recordset that will select the record based on the id. In the field in the recordset that holds the values you will need to explode the values into an array, just like you imploded the array to get the string of values but backwards. Once you get the string of values into an array you can then use the in_array function to compare values to values in this array to see if they are present.
The select list that you have should be populated by a different recordset. The record that is being updated should come from it's own recordset. So the select list on the page should have a loop for one recordset to populate the available options. Then you will be using the in array function with the variable that you set based on the values that were in the other recordset to compare against. So you will be comparing a value from the recordset that is populating the selections with the array of values you exploded earlier.
If you continue to have trouble with this please post back and include both pages in question so that I can get a feel for how you have them working now and what needs to be done to get them working correctly.
