in the code that creates the list, you need to use the explode() and in_array() function to convert the comma seperated list back into an array and check if the current list item is in that array.
in your previous post, you give the following code for writing the list options:
<option value="<?php echo $row_WADADynListcompanies_type['name']?>"<?php if (!(strcmp($row_WADADynListcompanies_type['name'], $row_WADAcompanies["category"]))) {echo "selected=\"selected\"";} ?>><?php echo $row_WADADynListcompanies_type['display']?></option>
you would need to change that to:
<option value="<?php echo $row_WADADynListcompanies_type['name']?>"<?php if(isset($row_WADAcompanies["category"]) && is_array(explode(", ", $row_WADAcompanies["category"]))) {
if(in_array($row_WADADynListcompanies_type['name'], explode(", ", $row_WADAcompanies["category"]))) echo(" selected=\"selected\"");
}
?>><?php echo $row_WADADynListcompanies_type['display']?></option>