jQuery post respond with readyState:0, status:0

all. I am using a jquery version 1.6.2. I do a ajax call like this:

  var jqxhr = $j.post(myPHP, function() {
          alert("success");
        })
        .success(function() { alert("second success"); })
        .error(function(data) { alert(JSON.stringify(data)); })
        .complete(function() { alert("complete"); });

The script, run, and show error, than, complete. Some people may thing that...myPHP have some problem, but the myPHP is always show:

{"sayHi":"hihi"}

So, I go to firebugs to check whether the link have any problem, when I called..: It show me the POST url with status 200, which is ok. Also, I can see the respond via firebugs.... But the question is ....why the jquery so me have a error...: And here is the error msg:

{"readyState":0,"responseText":"","status":0,"statusText":"error"}