html - How to send radio button value with AJAX and recover it with PHP -

i've been trying send value of radio button selected user , recover value php, problem can't recover value. here's code:

html

<form name="submission" action="">     <input type="radio" name="ex1" id="ex1_a" value="1">     <input type="radio" name="ex1" id="ex1_b" value="2">     <input type="radio" name="ex1" id="ex1_c" value="3">      <button class="buttons" type="submit">          submit     </button> </form> 

jquery script w/ ajax

$(function() {               $(".buttons").click(function() {              // validate , process form here            var radio_button_value;             if ($("input[name='ex1']:checked").length > 0){                radio_button_value = $('input:radio[name=ex1]:checked').val();            }            else{                alert("no button selected, try again!");                return false;            }            $.ajax({                type: "post",                 url: "save.php",                  data: radio_button_value,                  success: function() {                      alert("form submitted: "+ radio_button_value);                }             });             return false;          }); }); 

php

<?php          $selected_button = $_post['ex1'];          echo "test";          echo $selected_button; ?> 

the ajax part seems work since alert displayed, don't know if sending value correctly or if php wrong, echo "test" displayed, echo $selected_button never appears. appreciate help.

in ajax function have specify parameter name value:

$.ajax({                type: "post",                 url: "save.php",                  data: {"ex1":radio_button_value},                  success: function() {                      alert("form submitted: "+ radio_button_value);                }             });