Predefined Constants

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

CASE_LOWER (integer): 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 (integer): CASE_UPPER is used with array_change_key_case and is used to convert array keys to upper case.

Sorting order flags:

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

Sorting type flags: used by various sort functions

SORT_REGULAR (integer): SORT_REGULAR is used to compare items normally.
SORT_NUMERIC (integer): SORT_NUMERIC is used to compare items numerically.
SORT_STRING (integer): SORT_STRING is used to compare items as strings.
SORT_LOCALE_STRING (integer): SORT_LOCALE_STRING is used to compare items as strings, based on the current locale. Added in PHP 5.0.2.
SORT_NATURAL (integer): SORT_NATURAL is used to compare items as strings using "natural ordering" like natsort. Added in PHP 5.4.0.
SORT_FLAG_CASE (integer): SORT_FLAG_CASE can be combined (bitwise OR) with SORT_STRING or SORT_NATURAL to sort strings case-insensitively. Added in PHP 5.4.0.

Filter flags:

ARRAY_FILTER_USE_KEY (integer): ARRAY_FILTER_USE_KEY is used with array_filter to pass each key as the first argument to the given callback function. Added in PHP 5.6.0.
ARRAY_FILTER_USE_BOTH (integer): ARRAY_FILTER_USE_BOTH is used with array_filter to pass both value and key to the given callback function. Added in PHP 5.6.0.