If you are unsure whether enable or with shall be used, please
inquire on the mailing list.
WITH is solely for the purpose of pointing to paths and other external
entities.
Note that the comment has always mentioned "--disable-mbstring" which
clearly refers to enable and not with.
PHP_ARG_ENABLE(mbregex, whether to enable multibyte regex support,
[ --enable-mbregex Enable multibyte regex support], yes)
-PHP_ARG_WITH(mbstring, whether to enable multibyte string support,
+PHP_ARG_ENABLE(mbstring, whether to enable multibyte string support,
[ --disable-mbstring Disable multibyte string support], yes)
if test "$PHP_MBSTRING" != "no"; then