<?php

$arr = array(
	array(
		"category_name" => "Operating System",
		"sub_category_name" => "Windows 8"
	),
	array(
		"category_name" => "Operating System",
		"sub_category_name" => "Linux"
	),
	array(
		"category_name" => "Mobile",
		"sub_category_name" => "Nokia"
	)
);

$n = array();

$n = array();
$i = 0;

array_walk($arr, function($subArr) {
	global $n;
	global $i;
	$n[$subArr['category_name'] . $i] = $subArr['sub_category_name'];
	$i++;
});

var_dump($n);