You must define ALL - Chr: in this String
$phpdig_string_subst['iso-8859-7'] ='......here is iso-8859-7 chr ...........'
see:
http://www.softlab.ntua.gr/~sivann/xgrk/iso8859-7.html
and set:
define('PHPDIG_ENCODING','iso-8859-7');
Perhaps you found the code in ONE Line with google ?
-Roland-