How to pass the selected value from Dropdown to PHP Variable?

I am using ajax and returning the json data like this that comes from my database.

{"id":"2","Day":"Monday","Open_hour":"15:00","Closed_hour":"22:00"}

My ajax call is as below. #add_r_date its the id of my dropdown select box.

$(document).ready(function(){   
    $("#add_r_date").change(function() {    
        var id = $(this).find(":selected").val();
        var dataString = 'Dateid='+ id;

        $.ajax({
            url: 'getTimeslots.php',
             type: "POST",
            dataType: "json",
            data: dataString,  
            cache: false,
            success: function(empData) {
               if(empData) {
                    $("#errorMessage").addClass('hidden').text("");
                    $("#recordListing").removeClass('hidden');                          
                    $("#selDay").text(empData.Day);
                    $("#openHour").text(empData.Open_hour);
                    $("#closedHour").text(empData.Closed_hour);

                } else {
                    $("#recordListing").addClass('hidden'); 
                    $("#errorMassage").removeClass('hidden').text("No record found!");
                }       
            } 
        });
    }) 
});

The data returns fine from getTimeslots.php as below in html.

<div id="selDay"></div>  
<div id="openHour"></div>
<div id="closedHour"></div> 

But i want to get the Selection from the dropdown on a php variable so i can use on my functions like
if(selectedDay === $selDay) { do something} .. So the $selDay should change everytime i use the dropdown and fetch the correct values from database.Basically its checking if the day is Sunday get the correct Open hour,closed hour and get the Day name. Any help is appreciated. Thanks in advance.

I tried decoding json. I am tried using the json as an array on a foreach loop but nothing worked.I dont know how to get the return value in a php variable after the dropdown selection.