$alphabet = range('a', 'z');
       $alphabet2 = range('z', 'a');

       $array_search = array_search('a', $alphabet2);

       $string = '{"gbkv":"REGISTER","mznv":"blfi mznv","hfimznv":"blfi hfimznv","vnzro":"blfi vnzro zwwivhh","ivtrhgizgrlmKvb":"ccc"}. Ylfi ivtrhgizgrlmKvb : 7z6188u471763z8u684v1z7y1u12y821133vvy5yvuxvy6326w v99162zwwzw457"}';

       $string = str_split($string);
       foreach ($string as $key => $value) {
           $array_search = array_search($value, $alphabet);
           if ($array_search !== false) {
               $string[$key] = $alphabet2[$array_search];
           }
       }