jQuery检查一个远程、mysql一个表中的两个字段

下面是我的代码；

$(document).ready(function(){

var fieldA = $('#fieldA').attr('value');
var fieldB = $('#fieldB').attr('value');
    $('#simpleForm').validate({
    errorLabelContainer: "#error_message_box",
    wrapper: "li",
        rules: {                
            firstname: "required",
            fieldA: "required",
            fieldB: {
                                    required: true,
                remote: "chk.php",
                type: "POST",
                dataType: "json",
                data: "fieldA=" + fieldA + "fieldB=" + fieldB
                    }
        },
        messages: {                
            firstname: "Please enter your first name.",
                            fieldA: "Please enter field A.",
            fieldB: {
                                    required: "Please enter field B.",
                remote: "Field B already exist."
                                    }               
        }
    });
});

我无法检查字段A和字段B是否都存在于MySQL的同一个表中。

在PHP文件中只能得到fieldA，不能得到fieldB。