# 预定义常量

下列常量由此扩展定义，且仅在此扩展编译入 PHP 或在运行时动态载入时可用。

`MB_OVERLOAD_MAIL` (int)
  自 PHP 8.0.0 起已删除。
`MB_OVERLOAD_STRING` (int)
  自 PHP 8.0.0 起已删除。
`MB_OVERLOAD_REGEX` (int)
  自 PHP 8.0.0 起已删除。
`MB_CASE_UPPER` (int)
  Performs a full upper-case folding. This may change the length of the string. This is the mode used by mb_strtoupper().
`MB_CASE_LOWER` (int)
  Performs a full lower-case folding. This may change the length of the string. This is the mode used by mb_strtolower().
`MB_CASE_TITLE` (int)
  Performs a full title-case conversion based on the Cased and CaseIgnorable derived Unicode properties. In particular this improves handling of quotes and apostrophes. This may change the length of the string.
`MB_CASE_FOLD` (int)
  Performs a full case fold conversion which removes case distinctions present in the string. This is used for caseless matching. This may change the length of the string. Available since PHP 7.3.
`MB_CASE_LOWER_SIMPLE` (int)
  Performs a simple lower-case fold conversion. This does not change the length of the string. Available as of PHP 7.3.
`MB_CASE_UPPER_SIMPLE` (int)
  Performs simple upper-case fold conversion. This does not change the length of the string. Available as of PHP 7.3.
`MB_CASE_TITLE_SIMPLE` (int)
  Performs simple title-case fold conversion. This does not change the length of the string. Available as of PHP 7.3.
`MB_CASE_FOLD_SIMPLE` (int)
  Performs a simple case fold conversion which removes case distinctions present in the string. This is used for caseless matching. This does not change the length of the string. Used by case-insensitive operations internally by the MBString extension. Available as of PHP 7.3.
`MB_ONIGURUMA_VERSION` (string)
  Oniguruma 版本，例如 `6.9.4`。自 PHP 7.4 起可用。

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