SendAndLoad Problem

I simply used loadVars class, but seems like it can’t pass the parameter to PHP.

PHP
<?
echo"xx=$a&yy=5";
?>

*ActionScript *
myVar=new LoadVars();
myVar.a=1;
myVar.onLoad = function(success){
if(success){
trace(myVar.xx+" "+myVar.yy);
}
}
myVar.sendAndLoad(“test.php”,myVar,“post”);

I have tried this script on two servers.
the output from the first one was ‘1 5’, but the other came out ’ 5’.
which means the parameter ‘myVar.a=1’ didn’t pass to PHP.

So what’s wrong with the script or the second server.
Does anyone know how can I fix this problem?
Thanks.