<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'];
?>