使用http.send（）Ajax

if(document.getElementById(callerName).checked) {
    //alert(callerName);        
    var poststr = "field=" + escape(encodeURI(callerName)) +
      "&op=add" + "&nocache=" + nocache;
}
else {
    //alert(callerName);
    var poststr = "field=" + escape(encodeURI(callerName)) + 
      "&op=del" + "&nocache=" + nocache;
}
http.send(poststr);

When I recieve the $_POST['field'] i get '%20' where there are spaces..any solution to get exact the string?

PHP:

$field = urldecode($_POST['field']);

You are double-escaping your data by using both escape and encodeURI. Also, I'd recommend you use encodeURIComponent instead. Try changing to this:

var poststr = "field=" + encodeURIComponent(callerName) + 
  "&op=add&nocache=" + nocache;