在php中自定义数组

I have an array like this:

 Array ( [0] => Array 
             ([category_name] => Operating System
              [sub_category_name] => Windows 8) 
         [1] => Array 
             ([category_name] => Operating System
              [sub_category_name] => Linux)
         [2] => Array 
             ([category_name] => Mobile
              [sub_category_name] => Nokia))

I would like to customize it like this:

Array('Operating System' => 'Windows 8', 'Operating System' => 'Linux', 'Mobile' => 'Nokia')

Thanks in advance. Any help or suggestion would be a great help...

this may help

$result = array();
foreach($firstArray as $row) {
    $result[$row['category_name']] = $row['sub_category_name'];
}

if you want to merge duplicate keys you can write like this

$result = array();
foreach($firstArray as $row) {
    if(isset($result[$row['category_name']])) {
        if(!is_array($result[$row['category_name']])) {
            $tmp = array();
            $tmp[] = $result[$row['category_name']];
            $result[$row['category_name']] = $tmp;
        }
        $result[$row['category_name']][] = $row['sub_category_name'];
    } else 
    $result[$row['category_name']] = $row['sub_category_name'];
}

$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); // array(3) { ["Operating System0"]=> string(9) "Windows 8" ["Operating System1"]=> string(5) "Linux" ["Mobile2"]=> string(5) "Nokia" } 

NOTE: As OP updated question, I updated answer too, though in the same array the same keys are not permitted, so I added counter at the end of the key in the new array.

DEMO

PHP does not allow duplicate keys for same array.
If you want to use same keys for multiple values then you should use multidimensional array as mentioned in below example.

As per your above code below is your array 
$array = array("0" => array("category_name" => "Operating System", "sub_category_name" => "Windows 8"),
    "1" => array("category_name" => "Operating System", "sub_category_name" => "Linux"),
    "2" => array("category_name" => "Mobile", "sub_category_name" => "Nokia"));

you can merge the array and for same key use multidimensional array

$new_array = array();
foreach ($array as $arr) {

    $arrmerge = array($arr['category_name'] => $arr['sub_category_name']);
    if (array_key_exists($arr['category_name'], $new_array)) {
        $arrmerge[$arr['category_name']] = array($new_array[$arr['category_name']], $arr['sub_category_name']);
    }

    $new_array = array_merge($new_array, $arrmerge);
}
print_r($new_array);

I hope this will help you.