function signOut()
{
    if(!confirm('Ban co chac ban se thoat ra khong ?')) return false;
    
    var XMLhttp = getAjaxCore();
    
    
    // onreadystatechange
    XMLhttp.onreadystatechange  = function()
    {
        // prepare
        var inner       = document.getElementById('ajax_loading');
        
        var availWidth  = parseInt(document.documentElement.clientWidth)/2 - 300 + 'px';
        
        inner.style.left    = availWidth;
        inner.style.top     = 300 + 'px';
        
        inner.style.display = 'block';
        
        // case
        if(XMLhttp.readyState   != 4) {
            var img = '<center><img src="gui/ajax-loader3.gif"></center>'; 
            
            if(inner.innerHTML  != img) inner.innerHTML = img;
        }
        
        // catch return
        else {
            if(XMLhttp.responseText == 'true') { // succ
                
                var content = '<center>Đang chuyển hướng</center>';
                inner.innerHTML = content;
                
                location.href   = 'redirect.html';
                
            }  else { // catch error
                
                var content = '<center style="color:red;">Co loi xay ra.</center>';
                inner.innerHTML = content;
                
            }
            
        }
        
        
        
    }
    
    // sending information
    var url = 'Ajax_script/signout.ajax.php?tmp={random}';
    
    url     = url.replace('{random}', Math.random());
    
    
    XMLhttp.open('GET', url, true);
    XMLhttp.send(null);
    
}