Folding Menu - How To Remember Current Selection

I am new to JavaScript. I am currently developing a simple CMS in PHP & MySQL. Most of work is already finished, bu now I have to tweak some functionalities in increase user experience. One of it is folding menu (tree menu). I am generating it in PHP and currently I'm using simple JavaScript to expand/collapse submenus (folders). Idea is simple - when an expandable list item (<li>) is clicked (onmouseclick) a simple JS funtion is executed that display a child list (<ul>) and in this way I am building a simple tree menu.

The function is following:
Code:
function showHide(msg_id)
{
var element = document.getElementById(msg_id);
element.style.display = element.style.display=='block' ? 'none' : 'block';
}

The problem is that when I click an list item (that leads to certain article) in a submenu, whole page is reloaded and all previously opened submenus collapse. I would like to have expanded elements that are leading to the last one that I clicked. How can I achieve it? Can you provide me some links to tutorials? Or maybe giva a hint how to tweak my function?