Resource Types
Sorting Arrays
Arrays
PHP Manual

# 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 4.4.0 and 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.

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

Resource Types
Sorting Arrays
Arrays
PHP Manual