Via FTP, SSH, or Webby, change into the 'conf' directory, and create/edit a file called php.ini. You will have to first add in [PHP] at the top of the file, and then lines that are key-value pairs:
[PHP] key = value
[PHP] memory_limit = 64M expose_php = Off
If you are installing software that requests you modify your php.ini configuration you will likely be able to handle all of the necessary changes with this file.
A full list of php.ini options can be found at http://www.php.net/manual/en/ini.list.php