php wordpress api -- json conversion

gistfile1.php

<?php
header('Content-Type: application/json');
header('Access-Control-Allow-Origin: *');
require_once('wp-load.php');



$response = array();

if(isset($_GET['post_id'])) {
$args = array(
 'ID'                 => $_GET['post_id']

  );
$mypost = get_post($_GET['post_id']);
//echo json_encode($root_categories);
$response['mypost'] =$mypost;

} else if(isset($_GET['id'])) {
    // id index exists

if($_GET['id']==0){
$root_categories = get_categories( array(
	'parent' => 0,
) );
//echo json_encode($root_categories);
$response['cat'] =$root_categories;

}
else{
$args = array(
 'child_of'                 => $_GET['id']

  );
$root_categories = get_categories($args);
//echo json_encode($root_categories);
$response['cat'] =$root_categories;
$args = array( 'posts_per_page' => 15, 'offset'=> 1, 'category' =>  $_GET['id'] );

$myposts = get_posts( $args );
$response['myposts']=$myposts;
//echo json_encode($myposts);

}


}

echo json_encode($response);



?>