Modify array keys in PHP

I'm interested in functional programming lately and was happy to discover that modifying array keys in PHP can be done avoiding any kind of loops. 😃

$greetings = ['hey' => 1, 'ho' => 2];

$greetings = array_combine(array_map(function ($greeting) {
    return strtoupper($greeting);
}, array_keys($greetings)), array_values($greetings));

// ['HEY' => 1, 'HO' => 2]