get specific key value from array php

$ids = array_column($users, 'id');

$array = array(0 => 'blue', 1 => 'red', 2 => 'green', 3 => 'red');

$key = array_search('green', $array); // $key = 2;
$key = array_search('red', $array);   // $key = 1;

    function cmp($a, $b)
    {
        return strcmp($a->display_name, $b->display_name);
    }

    usort($blogusers, "cmp");

    foreach ($blogusers as $bloguser)
    {
        ...