Flattened a 3d array to a 2d array, then sorting rows by a column

Question

I have an array in following format:

array( 
    [0]=> array(
        [0]=> array ( 
            ["ID"]=> 118
            ["post_date"]=> "2014-04-28 07:27:37" 
            ["post_title"]=> "Title 1"
        )
        [1]=> array ( 
            ["ID"]=> 119
            ["post_date"]=> "2014-04-29 07:27:37" 
            ["post_title"]=> "title 2"
        )
    )
    [1]=> array(
        [0]=> array ( 
            ["ID"]=> 135
            ["post_date"]=> "2014-04-28 06:37:37" 
            ["post_title"]=> "Title 3"
        )
        [1]=> array ( 
            ["ID"]=> 148
            ["post_date"]=> "2014-04-25 07:27:37" 
            ["post_title"]=> "Title 4"
        )
    )
    [2]=> array(
        [0]=> array ( 
            ["ID"]=> 135
            ["post_date"]=> "2014-04-24 06:37:37" 
            ["post_title"]=> "Title 5"
        )
        [1]=> array ( 
            ["ID"]=> 148
            ["post_date"]=> "2014-04-25 09:21:37" 
            ["post_title"]=> "Title 6"
        )
    )
)

Now I need to sort this using post_date and show them in DESC order. This is in PHP.

I am not sure how to sort this. Can anybody please help me with this sorting?

This is how I want to show the output:

title 2
Title 1
Title 3
Title 6
Title 4
Title 5

Answer 1

Okay, I have solved it by flattening the array and then applying usort. This is how I have done it:

$query_results; //this is my main array
$flatten_array =array();
foreach ($query_results as $data) {
    foreach($data as $flatten_data) {
        $flatten_array[] = $flatten_data;
    }
}

function cpt_array_sort($a, $b) {
    return strtotime($b->post_date) - strtotime($a->post_date);
}

usort($flatten_array, 'cpt_array_sort');