You can do it by copy pasting these commands as root;
zypper ar -f http://download.opensuse.org/repositories/server:/php/openSUSE_13.1/server:php.repo
zypper ref
At this point it will ask you to "trust" the repository, press A to always trust the repository, it's maintained by openSUSE guys
zypper dup --from server_php
Restart Apache for the php 5.6.0 module to take effect.
Because the latest version released as update for 13.1 is indeed 5.4.20-30.1.
And it is openSUSE’s policy to not provide version upgrades in the official update repos.
Bug- and Security fixes are backported to the shipped version.
how can I upgrade to the latest php stable - 5.6.0?
You can add an additional repo and install it from there.
But why do you want to do that? Any specific reason?
If you are just worried about bugs or security problems:
You are not really using 5.4.20, a lot of bug and security fixes have been added by (open)SUSE.
Have a look at the package changelog:
PHP 5.6.0 has vastly better performance when combined with php5-opcache by the way, that’s one really good reason to upgrade it and use php5-opcache module