xmlHttp.open("GET","for.php?text="+url,true); 

xmlHttp.open("POST","for.php",true); 
xmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");

<script src="a.js" type="text/javascript"></script>
<a href="#" onClick="funphp100('o')">o</a>
<a href="#" onClick="funphp100('t')">t</a>
<a href="#" onClick="funphp100('x')">x</a>
<div id="php100"></div>

var xmlHttp;    
function S_xmlhttprequest(){ 
 if(window.ActiveXObject){ 
 xmlHttp=new ActiveXObject('Microsoft.XMLHTTP');
 }else if(window.XMLHttpRequest){ 
  xmlHttp=new XMLHttpRequest();
  }
 }
 
function funphp100(n){
var data = "text=" +n;　　//多个参数的，往后加
 S_xmlhttprequest();
xmlHttp.open("POST","for.php",true); 
xmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
 xmlHttp.onreadystatechange=byphp;
 xmlHttp.send(data);
 }
 
function byphp(){
var byphp100=xmlHttp.responseText;
document.getElementById("php100").innerHTML=byphp100;
 }

<?
echo $_POST['text'];
?>

var xmlHttp;    
function S_xmlhttprequest(){ 
 if(window.ActiveXObject){ 
 xmlHttp=new ActiveXObject('Microsoft.XMLHTTP');
 }else if(window.XMLHttpRequest){ 
  xmlHttp=new XMLHttpRequest();
  }
 }
 
 
function funphp100(url){
 S_xmlhttprequest();
xmlHttp.open("GET","for.php?text="+url,true); 
 xmlHttp.onreadystatechange=byphp; 
 xmlHttp.send(null);
 }
 
function byphp(){
var byphp100=xmlHttp.responseText;
document.getElementById("php100").innerHTML=byphp100;
 }

<?
echo $_GET['text'];
?>