function array_replace_key($search, $replace, array $subject) {
    $updatedArray = [];

    foreach ($subject as $key => $value) {
        if (!is_array($value) && $key == $search) {
            $updatedArray = array_merge($updatedArray, [$replace => $value]);
            
            continue;
        }

        $updatedArray = array_merge($updatedArray, [$key => $value]);
    }

    return $updatedArray;
}

$user = ['name' => 'Donald Pakkies', 'age' => 22];

var_dump(array_replace_key('name', 'full_name', $user));