AJAX POST傳值

function showHint(str){
  var http = new XMLHttpRequest();
  if (str.length==0){
    document.getElementById("txtHint").innerHTML="";
    return;
  }
  var params = "txt1="+str;
  http.open("POST","posthint.php",true);
  http.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  http.setRequestHeader("Content-length", params.length);
  http.setRequestHeader("Connection", "close");
  http.onreadystatechange = function() {
    if(http.readyState == 4 && http.status == 200) {
      document.getElementById("txtHint").innerHTML=http.responseText;
    }
  }
  http.send(params);
}

首先要先建立一個 http 請求對象

var http = new XMLHttpRequest();

設定要向Server請求的參數(2個以上用"&"連接)

var params = "txt1="+str;

設定傳值的方式和後端網頁

http.open("POST","posthint.php",true);

POST 請求必須設置的 http header

http.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
http.setRequestHeader("Content-length", params.length);
http.setRequestHeader("Connection", "close");


檢查請求目前的狀態和Server回傳的狀態

http.onreadystatechange = function() {
  if(http.readyState == 4 && http.status == 200) {
    document.getElementById("txtHint").innerHTML=http.responseText;
  }
}

將請求發送給 Server

http.send(params);

接下來html的部分就是輸入後去執行上述代碼
PHP的部分只要將送出的參數處理後用echo印出來即可

相關參考：

http://www.openjs.com/articles/ajax_xmlhttp_using_post.php