Displaying Tooltip When Hover Over Link

I'm trying to make script that displays a tooltip when you hover over something, like a link. But it doesn't work and I have no idea why.

This is my script:
set the function that will draw the tooltip
function tooltip(header, content, w, h) {
get the tooltip element
var tooltip = document.getElementById('tooltip');
set the browser variable
var browser;
check what browser the use is using
if (document.all) browser = 'ie'; Internet Explorer
else if(document.layers) browser = 'ns4'; Netscape 4+
else if(window.navigator.userAgent.toLowerCase().match('gecko')) browser = 'gecko'; Mozilla

set the mouse coordinates
var x = event.clientX;
var y = event.clientY;
change the tooltip's width
tooltip.style.width = w;
change the tooltip's height
tooltip.style.height = h;
check if the header is empty
if (header.length = 0) header = 'Help';
change the content in the tooltip
tooltip.innerHTML = header + '<br>' + content;

change the visibility
tooltip.style.visibility = 'visible';
set where to draw the tooltip
tooltip.style.position = 'absolute';
tooltip.style.top = y;
tooltip.style.left = x;
tooltip.style.zIndex = 9999999;
} set the function that hides the tooltip
function hide_tooltip() {
get the element
var tooltip = document.getElementById('tooltip');
hide the tooltip
tooltip.style.visibility = 'hidden';
}

This how I'm using it on link:
<a href='java script: void(0)' onmouseover='tooltip("","This is a tooltip",100,50)' onmouseout='hide_tooltip()'>?</a>