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Use of undefined constant JSON_UNESCAPED_SLASHES - assumed 'JSON_UNESCAPED_SLASHES'
Use of undefined constant JSON_UNESCAPED_UNICODE - assumed 'JSON_UNESCAPED_UNICODE'
Use of undefined constant JSON_HEX_TAG - assumed 'JSON_HEX_TAG' in /home/jidanni/me
Use of undefined constant JSON_HEX_AMP - assumed 'JSON_HEX_AMP' in /home/jidanni/me

seen in git as of
$ date -u
Tue Jun  4 12:41:24 UTC 2013

What version of PHP?

$ dpkg -l \*php*|grep ^ii
ii  libapache2-mod-php5   5.5.0~rc2+dfsg- i386            server-side, HTML-embedded scripting language (A
ii  php-doc               20100521-2      all             Documentation for PHP5
ii  php-elisp             1.5.0-1.1       all             Emacs support for php files
ii  php-gettext           1.0.11-1        all             read gettext MO files directly, without requirin
ii  php5                  5.5.0~rc2+dfsg- all             server-side, HTML-embedded scripting language (m
ii  php5-cgi              5.5.0~rc2+dfsg- i386            server-side, HTML-embedded scripting language (C
ii  php5-cli              5.5.0~rc2+dfsg- i386            command-line interpreter for the php5 scripting 
ii  php5-common           5.5.0~rc2+dfsg- i386            Common files for packages built from the php5 so
ii  php5-mcrypt           5.5.0~rc2+dfsg- i386            MCrypt module for php5
ii  php5-mysql            5.5.0~rc2+dfsg- i386            MySQL module for php5
ii  phpmyadmin            4:4.0.1-2       all             MySQL web administration tool

json_encode has been around for a while, and we use it in the code paths for older PHP, just with some different options set where they are available.

It would sound like your current version is missing the json support for some reason or another.

As you're running PHP 5.5 rc2, I'd contact whoever is your upstream package provider and tell them of the issue. I can't really believe this is MediaWiki at fault, and as 5.5 rc2 came out a couple of weeks ago...

Oops...
php5 (5.5.0~rc1+dfsg-1) experimental; urgency=low

  * Starting from this version, the JSON module is no longer compiled in
    due to licensing problems, and you need to install the JSON extension
    as external php5-json package that is using json-c library.

 -- Ondřej Surý <ondrej@debian.org>  Fri, 17 May 2013 14:43:04 +0200

And the Debian bug is http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=692613 :

 % rgrep 'The Software shall be used for Good, not Evil.' .
 ./README.REDIST.BINS:The Software shall be used for Good, not Evil.
 ./ext/json/utf8_to_utf16.c:The Software shall be used for Good, not Evil.
 ./ext/json/utf8_decode.c:The Software shall be used for Good, not Evil.
 ./ext/json/JSON_parser.c:The Software shall be used for Good, not Evil.


:-D

See bug 49122.

*** This bug has been marked as a duplicate of bug 49122 ***