handle json request in PHP

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) {}
});