<script type="text/javascript">
function selectAll(element)
{
    for(var i=0;i<element.form.elements.length;i++)
    {
        if(element.form.elements[i].getAttribute('type').toLowerCase=='checkbox')
        {
            element.form.elements[i].checked=element.checked;
        }
    }
}
</script>


<form>
<input type="checkbox" onclick="selectAll(this)"/>
<input type="checkbox" name="el_name[]">
<input type="checkbox" name="el_name[]">
<input type="checkbox" name="el_name[]">
<input type="checkbox" name="el_name[]">
<input type="checkbox" name="el_name[]">
</form>