Convert array keys from underscore_case to camelCase recursively

Question

I had to come up with a way to convert array keys using undescores (underscore_case) into camelCase. This had to be done recursively since I did not know what arrays will be

Answer 1

The recursive part cannot be further simplified or prettified.

But the conversion from underscore_case (also known as snake_case) and camelCase can be done in several different ways:

$key = 'snake_case_key';
// split into words, uppercase their first letter, join them, 
// lowercase the very first letter of the name
$key = lcfirst(implode('', array_map('ucfirst', explode('_', $key))));

or

$key = 'snake_case_key';
// replace underscores with spaces, uppercase first letter of all words,
// join them, lowercase the very first letter of the name
$key = lcfirst(str_replace(' ', '', ucwords(str_replace('_', ' ', $key))));

or

$key = 'snake_case_key':
// match underscores and the first letter after each of them,
// replace the matched string with the uppercase version of the letter
$key = preg_replace_callback(
    '/_([^_])/',
    function (array $m) {
        return ucfirst($m[1]);
    },
    $key
);

Pick your favorite!