All text in MediaWiki's database uses MySQL's "BINARY" encoding by
default, even though it's encoded in UTF-8, for historical reasons
(MySQL's UTF-8 support used to be horribly broken).
The software you're using to view it therefore decides to show you the
binary bytes, rather than the actual text. I've seen this annoying
behavior in some versions of PHPMyAdmin and MySQLWorkbench, I don't know
what you're using.
You can "decode" the value '68696464656e636174' based on the ASCII
values for the hex codes:
0x68 = 'h'
0x69 = 'i'
0x64 = 'd'
0x64 = 'd'
0x65 = 'e'
0x6e = 'n'
0x63 = 'c'
0x61 = 'a'
0x74 = 't'
(
https://en.wikipedia.org/wiki/ASCII#Printable_characters)
Or, you can cast it to the 'char' type in your query to make your
software behave:
SELECT CAST(pp_propname as char) FROM page_props;
The actual data stored in your database has been 'hiddencat' all along,
and MediaWiki doesn't need to do anything special to get that. The
problem is with the viewer software being too clever.
--
Bartosz Dziewoński