/usr/include/php5/ext/mbstring/ is in php5-dev 5.5.9+dfsg-1ubuntu4.
This file is owned by root:root, with mode 0o755.
..
/usr/include/php5/ext/mbstring/libmbfl/
/usr/include/php5/ext/mbstring/libmbfl/config.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/
/usr/include/php5/ext/mbstring/libmbfl/mbfl/eaw_table.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfilter.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfilter_8bit.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfilter_pass.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfilter_wchar.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfl_allocators.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfl_consts.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfl_convert.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfl_defs.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfl_encoding.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfl_filter_output.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfl_ident.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfl_language.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfl_memory_device.h
/usr/include/php5/ext/mbstring/libmbfl/mbfl/mbfl_string.h
/usr/include/php5/ext/mbstring/mbstring.h
/usr/include/php5/ext/mbstring/oniguruma/
/usr/include/php5/ext/mbstring/oniguruma/oniguruma.h
/usr/include/php5/ext/mbstring/php_mbregex.h
/usr/include/php5/ext/mbstring/php_onig_compat.h