* SORT_NUMERIC - keys will be compared as numbers;
* SORT_STRING - keys will be compared as strings. If intl extension is enabled, then comparison will be language-sensitive using internally a created ICU locale;
* SORT_LOCALE_STRING - keys will be compared as strings depending on the current POSIX locale. If intl extension is enabled, then comparison will be language-sensitive using internally a created ICU locale.
* This function is aimed at replacing the function ksort() for sorting human-language key strings.