jQuery,Ajax,JSON,PHP和parsererror

我正在尝试使用$.ajax在 jQuery中将表单发送到PHP.我将整个内容发送为JSON,但是,当我尝试获取响应时,我得到’parsererror’.我究竟做错了什么？

jQuery片段：

$("form").submit(function(){
    $.ajax({type: "POST",url: "inscrever_curso.PHP",data: {Cpf : $("input#cpf").val(),nome : $("input#nome").val()},dataType: "json",success: function(data){
            alert("sucesso: "+data.msg);
        },error: function(XMLhttprequest,textStatus,errorThrown){
            alert ("erro: "+textStatus);
        }
    });
    return false;
});

PHP：

<?PHP
    $return[msg]="TesTing,tesTing.";
    echo json_encode($return);
?>

$return[msg]="TesTing,tesTing.";
echo json_encode($return);

Notice: Use of undefined constant msg - assumed 'msg'

$return['msg']="TesTing,tesTing.";
echo json_encode($return);