$action='a';

if($action=='a')
$uu=$_POST['username'];

session_start();

$_SESSION['action']  = 'a';
    $_SESSION['Username']  = $_POST['username'];

$username = $_SESSION['Username'];
    echo $_SESSION['Username'];

//Set $_SESSION to a new array(), overwriting the old one
    $_SESSION = array();
    //Make the cookie expired if the cookie is used when you create the session
    if (isset($_COOKIE[session_name()])) {
        //Set the time of the cookie to the past, so it will expired immediately. Can't remember why I use 42000, but It was recommended by a site
        setcookie(session_name(), '', time()-42000, '/');
    }
    //Finally destry the who session information
    session_destroy();

<style type="text/css">

#hintbox{ /*CSS for pop up hint box */
position:absolute;
top: 0;
background-color: lightyellow;
width: 150px; /*Default width of hint.*/
padding: 3px;
border:1px solid black;
font:normal 11px Verdana;
line-height:18px;
z-index:100;
border-right: 3px solid black;
border-bottom: 3px solid black;
visibility: hidden;
}

.hintanchor{ /*CSS for link that shows hint onmouseover*/
font-weight: bold;
color: navy;
margin: 3px 8px;
}

</style>

<script type="text/javascript">

/***********************************************
* Show Hint script- © Dynamic Drive (www.dynamicdrive.com)
* This notice MUST stay intact for legal use
* Visit http://www.dynamicdrive.com/ for this script and 100s more.
***********************************************/
        
var horizontal_offset="9px" //horizontal offset of hint box from anchor link

/////No further editting needed

var vertical_offset="0" //horizontal offset of hint box from anchor link. No need to change.
var ie=document.all
var ns6=document.getElementById&&!document.all

function getposOffset(what, offsettype){
var totaloffset=(offsettype=="left")? what.offsetLeft : what.offsetTop;
var parentEl=what.offsetParent;
while (parentEl!=null){
totaloffset=(offsettype=="left")? totaloffset+parentEl.offsetLeft : totaloffset+parentEl.offsetTop;
parentEl=parentEl.offsetParent;
}
return totaloffset;
}

function iecompattest(){
return (document.compatMode && document.compatMode!="BackCompat")? document.documentElement : document.body
}

function clearbrowseredge(obj, whichedge){
var edgeoffset=(whichedge=="rightedge")? parseInt(horizontal_offset)*-1 : parseInt(vertical_offset)*-1
if (whichedge=="rightedge"){
var windowedge=ie && !window.opera? iecompattest().scrollLeft+iecompattest().clientWidth-30 : window.pageXOffset+window.innerWidth-40
dropmenuobj.contentmeasure=dropmenuobj.offsetWidth
if (windowedge-dropmenuobj.x < dropmenuobj.contentmeasure)
edgeoffset=dropmenuobj.contentmeasure+obj.offsetWidth+parseInt(horizontal_offset)
}
else{
var windowedge=ie && !window.opera? iecompattest().scrollTop+iecompattest().clientHeight-15 : window.pageYOffset+window.innerHeight-18
dropmenuobj.contentmeasure=dropmenuobj.offsetHeight
if (windowedge-dropmenuobj.y < dropmenuobj.contentmeasure)
edgeoffset=dropmenuobj.contentmeasure-obj.offsetHeight
}
return edgeoffset
}

function showhint(menucontents, obj, e, tipwidth){
if ((ie||ns6) && document.getElementById("hintbox")){
dropmenuobj=document.getElementById("hintbox")
dropmenuobj.innerHTML=menucontents
dropmenuobj.style.left=dropmenuobj.style.top=-500
if (tipwidth!=""){
dropmenuobj.widthobj=dropmenuobj.style
dropmenuobj.widthobj.width=tipwidth
}
dropmenuobj.x=getposOffset(obj, "left")
dropmenuobj.y=getposOffset(obj, "top")
dropmenuobj.style.left=dropmenuobj.x-clearbrowseredge(obj, "rightedge")+obj.offsetWidth+"px"
dropmenuobj.style.top=dropmenuobj.y-clearbrowseredge(obj, "bottomedge")+"px"
dropmenuobj.style.visibility="visible"
obj.onmouseout=hidetip
}
}

function hidetip(e){
dropmenuobj.style.visibility="hidden"
dropmenuobj.style.left="-500px"
}

function createhintbox(){
var divblock=document.createElement("div")
divblock.setAttribute("id", "hintbox")
document.body.appendChild(divblock)
}

if (window.addEventListener)
window.addEventListener("load", createhintbox, false)
else if (window.attachEvent)
window.attachEvent("onload", createhintbox)
else if (document.getElementById)
window.onload=createhintbox

</script>