Encoding JSON in PHP
Some years ago, JSON support was provided through the json pecl extension. Since PHP 5.2, it is included in the core directly, so if you use a recent PHP version you should have no trouble using it.

Note: If you run an older version of PHP than 5.3, I recommend you to upgrade anyway. PHP 5.3 is the oldest version that is currently supported and with the latest PHP security bugs found I would consider it critical to upgrade as soon as possible.

Step 1: Create file with the naming as index.php

<?php 
 
 
$username = 'root';  //database user name variable
$password ='';       //database password
$host ='localhost';

$databasename = 'json-db';

 $conn = mysql_connect($host, $username, $password);
 if (!$conn)
    {
  die('Could not connect: ' . mysql_error());
 }
//select database name and connect 

 mysql_select_db($databasename, $conn);

 
 $sth = mysql_query("Select * from users");
 $rows = array();
 while($r = mysql_fetch_assoc($sth)) {
  $rows[] = $r;
 }
 $json = json_encode($rows);
 echo stripslashes($json); 
?>

Step 2: Retrieve JSON data (callback.php)
There are two ways to call data from another file (index.php), using JQuery or PHP (json_decode).

a. Using JQuery

<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script>
$(document).ready(function (){
        //using getJSON
 $.getJSON("http://localhost/path-to-your-file/index.php", function(data){ 
    // do stuff with json (in this case an array)
    alert("Success");   
 });
 
 //using Ajax
 $.ajax({
   url:"http://127.0.0.1:8888/json/index.php",
   dataType: 'json', // Notice! JSONP <-- P (lowercase)
   success:function(json){
    // do stuff with json (in this case an array)
    alert("Success");
   },
   error:function(){
    alert("Error");
   },
 });
});
</script>

b. Using PHP json_decode()

<?php
$url = 'http://localhost/path-to-your-file/index.php';
$json = file_get_contents($url);
$datas = json_decode($json);
//var_dump($datas);

foreach($datas as $data){
 echo $data->last_name.'
';
}
?>

credited to: http://nitschinger.at/Handling-JSON-like-a-boss-in-PHP