mcrypt_list_modes() is used to get an array of all supported modes in the lib_dir.
mcrypt_list_modes() takes as optional parameter a directory which specifies the directory where all modes are located. If not specifies, the value of the mcrypt.modes_dir php.ini directive is used.
The above example will produce a list with all supported algorithms in the default mode directory. If it is not set with the ini directive mcrypt.modes_dir, the default directory of mcrypt is used (which is /usr/local/lib/libmcrypt).