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.

COUNT_NORMAL (int)
COUNT_RECURSIVE (int)
EXTR_OVERWRITE (int)
EXTR_SKIP (int)
EXTR_PREFIX_SAME (int)
EXTR_PREFIX_ALL (int)
EXTR_PREFIX_INVALID (int)
EXTR_PREFIX_IF_EXISTS (int)
EXTR_IF_EXISTS (int)
EXTR_REFS (int)