[horde] Upgrade PHP 5.4.45 to 5.6.31

[Anton Köstlbacher]

Hi Horders,

I tried updating a Debian Wheezy PHP from 5.4.45-0+deb7u9 to 
5.6.31-1~dotdeb+7.1

After upgrading PHP, other apps like Mediakwiki, Wordpress etc. are 
working just fine.

When logging into Horde I get:

2017-08-10T22:25:26+02:00 EMERG: HORDE Horde_Prefs_Scope::serialize() 
must return a string or NULL [pid 21634 on line 62 of 
"/usr/share/php/Horde/Prefs/Cache/HordeCache.php"]

I disabled all caching, but it doesn't help. There were changes in PHP 
regarding serialization and also json_decode. Could there be serialized 
values in hordes prefs table which need to be updated/corrected before 
upgrading PHP?

Sorry if this was asked before, but I couldn't find anything. Thanks for 
your help!

Kind Regards,
Anton