Permissions/configuring apache so directory can have files included from but php cannot fopen

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http://serverfault.com – Is there a way to set permissions on a directory or configure apache in a way that allows PHP to include files from this directory for execution but scripts cannot open them using fopen() or other similar functionality? I am trying to secure some keys, salts and whatnot that I don't want scripts to be able to reference or read directly. (HowTos)