When making an ajax call, when contentType is set to application/json instead of the default x-www-form-urlencoded, server side (in PHP) can't get the post parameters.
in the following working example, if I set the contentType to "application/json" in the ajax request, PHP $_POST would be empty. why does this happen? How can I handle a request where contentType is application/json properly in PHP?

     
   $.ajax({
                    cache: false,
                    type: "POST",
                    url: "xxx.php",
                    //contentType: "application/json",
                    processData: true,
                    data: {my_params:123},
                    success: function(res){

                    },
                    complete: function(XMLHttpRequest, text_status) {

                    }
                  });