jQuery $.ajax() does not seem to work correctly with IE8 but it is working with Firefox, Chrome and Safari. I am submittin the form and response back in JSO
Your JSON response (output of _test.php) should contain : instead of ;, as @sje397 said. Seems like other browser are okay with ;, but not IE8, so the value is kept a string instead being parsed as an object.
EDIT: for some reason your data isn't being converted to json, try putting
if(typeof(data)=='string') {
data=$.parseJSON(data);
}
at the top of your success handler and see if it works.