You can leverage the Jessie repository to upgrade certain Wheezy packages, but all the usual admonitions apply. Testing this on a backup machine is a really good idea,
Copy /etc/apt/sources.list > /etc/apt/sources.list.wheezy
Edit the /etc/apt/sources.list file:
gedit /etc/apt/sources.list
and replace the contents of /etc/apt/sources.list
with the following lines:
deb h.t.t.p://ftp.de.debian.org/debian/ jessie main contrib non-free
deb-src h.t.t.p://ftp.de.debian.org/debian/ jessie main contrib non-free
deb h.t.t.p://httpredir.debian.org/debian jessie-updates main contrib non-free
deb-src h.t.t.p://httpredir.debian.org/debian jessie-updates main contrib non-free
deb h.t.t.p://security.debian.org/ jessie/updates main contrib non-free
deb-src h.t.t.p://security.debian.org/ jessie/updates main contrib non-free
(Change h.t.t.p > http. You can replace "de" with e.g. "us" or your country code, but it should make no difference).
Save the file and run the following command to update the sources database:
apt-get update
DO NOT RUN apt-get upgrade!
Next,
apt-get install phpmyadmin
choose your options. I didn't modify my config but did upgrade the database.
Then, just to be safe, I ran
php5enmod mcrypt
service php5-fpm restart
IMPORTANT!!! Change your repository back to wheezy!
Rename /etc/apt/sources.list to /etc/apt/sources.list.jessie
Rename /etc/apt/sources.list.wheezy to /etc/apt/sources.list
Run
apt-get update
You should be back on the Wheezy repositories.
That should do it.