在javascript if语句中取消设置$_POST

我需要取消设置$_POST['u']，但仅在javascript函数内，现在$_POST ARRAY在页面准备好后自动取消设置。。。

function resetChat(cod_c){
            if(confirm('are you sure?')){
                if (sendReq.readyState == 4 || sendReq.readyState == 0) {
                    sendReq.open("POST", 'getChat.php?chat='+cod_c+'&last=' + lastMessage, true);
                    sendReq.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
                    sendReq.onreadystatechange = handleResetChat; 
                    var param = 'action=reset';
                    param += '&user=' + document.getElementById('cod_user').value;
                    sendReq.send(param);
                    document.getElementById('txt_message').value = '';
                }
            <?php  unset($_POST['u']); ?>  // this only 1 time after confirm???
            }                       
        }
function handleResetChat() {
        }