Programming :: POST Method And PHP - Login Verification

I wrote a code for login verification..I got output with GET. But I need output with POST since it is more secure. Any error in my code.

javascript code:
var xml;
function verifyusernamepasswd(pass)
{ //pass is password that will be passed as parameter
xml=new XMLHttpRequest();
var url="http://localhost/loginvalidate.php";
var para="q="+username+"&p="+pass;//username is global
xml.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xml.setRequestHeader("Content-length", para.length);
xml.setRequestHeader("Connection", "close");
xml.open("POST",url,true);
xml.onreadystatechange=statechanged1;
xml.send(para);
} .....

The verification does not return anything, cos my alert is not displayed at all.