Predefined Constants

The constants below are always available as part of the PHP core.

CASE_LOWER (int): CASE_LOWER is used with array_change_key_case() and is used to convert array keys to lower case. This is also the default case for array_change_key_case().
CASE_UPPER (int): CASE_UPPER is used with array_change_key_case() and is used to convert array keys to upper case.

Sorting order flags:

SORT_ASC (int): SORT_ASC is used with array_multisort() to sort in ascending order.
SORT_DESC (int): SORT_DESC is used with array_multisort() to sort in descending order.

Sorting type flags: used by various sort functions

SORT_REGULAR (int): SORT_REGULAR is used to compare items normally.
SORT_NUMERIC (int): SORT_NUMERIC is used to compare items numerically.
SORT_STRING (int): SORT_STRING is used to compare items as strings.
SORT_LOCALE_STRING (int): SORT_LOCALE_STRING is used to compare items as strings, based on the current locale.
SORT_NATURAL (int): SORT_NATURAL is used to compare items as strings using "natural ordering" like natsort().
SORT_FLAG_CASE (int): SORT_FLAG_CASE can be combined (bitwise OR) with SORT_STRING or SORT_NATURAL to sort strings case-insensitively.

Filter flags:

ARRAY_FILTER_USE_KEY (int): ARRAY_FILTER_USE_KEY is used with array_filter() to pass each key as the first argument to the given callback function.
ARRAY_FILTER_USE_BOTH (int): ARRAY_FILTER_USE_BOTH is used with array_filter() to pass both value and key to the given callback function.