Installation

mbstring is a non-default extension. This
means it is not enabled by default. You must explicitly enable the
module with the configure option. See the Install section for details.

The following configure options are related to
the mbstring module.

• –enable-mbstring : Enable
  mbstring functions. This option is required to use
  mbstring functions.

  libmbfl is
  necessary for mbstring. libmbfl is bundled with mbstring.
  Before PHP 7.3.0, if libmbfl is
  already installed on the system, –with-libmbfl[=DIR] can be specified
  to use the installed library.

• –disable-mbregex : Disable regular
  expression functions with multibyte character support.

  Oniguruma is
  necessary for the regular expression functions with multibyte
  character support. Oniguruma is
  bundled with mbstring. As of PHP 5.4.0, if Oniguruma is already installed on the system,
  –with-onig[=DIR] can be
  specified to use the installed library.

  As of PHP 5.4.0 it is possible to disable the
  multibyte regex backtrack check by specifying –disable-mbregex-backtrack .