jQuery: sending JSON data to PHP with AJAX

 Passing JSON data with jQuery and AJAX to Node.js is relatively simple, passing JSON data with jQuery and AJAX to PHP requires a few extra steps.

Instead of trying to send JSON as is, you should only send a well-formed JSON string and let PHP to transform it into an object or an associative array (depending on how you use the json_decode() function).

Bear in mind that PHP not only requires the string to be valid JSON: your string must also be UTF-8 compliant. So with jQuery we can write:

var data = {

  test: $( "#test" ).val()

};

var options = {

  url: "test.php",

  dataType: "text",

  type: "POST",

  data: { test: JSON.stringify( data ) }, // Our valid JSON string

  success: function( data, status, xhr ) {

     //...

  },

  error: function( xhr, status, error ) {

      //...

  }

};

$.ajax( options );

And with PHP we have to take care of the string encoding before using it:

<?php

  header('Content-Type: text/plain');

  $test = utf8_encode($_POST['test']); // Don't forget the encoding

  $data = json_decode($test);

  echo $data->test;

  exit();

 ?>

Reff : https://gabrieleromanato.name/jquery-sending-json-data-to-php-with-ajax