function plannerCloseGroup(groupID){
    // retrieve all items having a property group with value groupID
    var itemsFiltered = items.get({
      filter: function (item) {
        return (item.group == groupID);
      }
    });
    closedItems[groupID] = itemsFiltered;
    items.remove(itemsFiltered);
    $("#plannerMinus_" + groupID).addClass("hidden");
    $("#plannerMinus_" + groupID).removeClass("display");
    $("#plannerPlus_" + groupID).removeClass("hidden");
    $("#plannerPlus_"+ groupID).addClass("display");
}

function plannerOpenGroup(groupID){
    items.update(closedItems[groupID]);
    $("#plannerMinus_" + groupID).addClass("display");
    $("#plannerMinus_" + groupID).removeClass("hidden");
    $("#plannerPlus_" + groupID).removeClass("display");
    $("#plannerPlus_" + groupID).addClass("hidden");
    delete closedItems[groupID];
}

'<span id="plannerPlus_'.$employeeID.'" class="hidden"><img src="images/planner-plus-grey.gif" width=10 height=10 onclick="plannerOpenGroup('.$employeeID.')"> </span><span id="plannerMinus_'.$employeeID.'" class="display"><img src="images/planner-minus.gif" width=10 height=10 onclick="plannerCloseGroup('.$employeeID.')"> </span>'