Select Multiple on Form Dropdown

I need some assistance changing a single select into a multiple select form drop-down. I am having issues with the multiple selections storing. Thank you for your help!

                $var = "

                <script type='text/javascript'>

                jQuery(function(){

                    

                         jQuery('form').attr('method', 'get');

                         jQuery('form').after('<input type=\"submit\"  id=\"doaction_k2\" class=\"button-secondary action\" value=\"Apply Folder Access\" style=\"background: red; color: #fff;\">');

                         

                

                    var thisurl = '';

                    var thaturl = '';

                    jQuery('#doaction_k2').live('click', function(){

                           jQuery('.new_file_access').each(function(){

                            thisurl = thisurl + '|' + jQuery(this).attr('userid') + '~' + jQuery(this).val();

                           });

                           

                           jQuery('.new_file_readwrite').each(function(){

                            thaturl = thaturl + '|' + jQuery(this).attr('userid') + '~' + jQuery(this).val();

                           });

                        

                        jQuery.post(document.URL, { update_access: 1, file_access: thisurl, file_readwrite: thaturl }, function(){

                            window.location = document.URL;

                        });

                        

                        return false;

                        });

                    });

                    

                </script>";

            }

                

                $var .= "<select multiple='yes' id='file_manager_accessk".$user_id."[]' class='new_file_access' userid='$user_id'><option value='' $noaccess>No Access</option>";
                
                foreach($ACCESS_ARRAY as $key=>$value)

                {

                        $key = str_replace('\\', '/', $key);

                        $keys = $key .'~'. $urls.'/'.$value;

                        $keys = substr($keys, 0, strlen($keys)-1);

                        $keys1 = base64_encode($keys);

                        

                        $var .= "<option value='$keys1' $select[$keys]>$value</option>";

                }

                $var .= "</select>";

                return $var;

~ Sarah