Reinstall Apache2 failled after upgrading php to 5.6

Hello,

Learning by doing/reading is what I want. Failling is also part of the game..

I realized that installing php5 was installing php 5.3. I was looking for a newer version and the only way was to install php from testing repo (following this tutorial http://www.binarytides.com/enable-testing-repo-debian/ ). I obviously messed up since apache2 wasn't able to start (apache got updated).
So I decided to roll back, remove php5.6 and install back the original one... but.. it seems not to work as expected

I get the error : command symlink_to_dir is unknown

# apt-get --reinstall install apache2
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
  apache2-mpm-prefork apache2-utils apache2.2-common
Suggested packages:
  www-browser apache2-doc apache2-suexec apache2-suexec-custom
Recommended packages:
  ssl-cert
The following NEW packages will be installed:
  apache2-mpm-prefork apache2-utils apache2.2-common
0 upgraded, 3 newly installed, 1 reinstalled, 0 to remove and 5 not upgraded.
2 not fully installed or removed.
Need to get 465 kB of archives.
After this operation, 944 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://apt.readynas.com/packages/readynasos/ 6.2.2/main apache2-utils armel 2.2.29-0.netgear2 [170 kB]
Get:2 http://apt.readynas.com/packages/readynasos/ 6.2.2/main apache2.2-common armel 2.2.29-0.netgear2 [291 kB]
Get:3 http://apt.readynas.com/packages/readynasos/ 6.2.2/main apache2-mpm-prefork armel 2.2.29-0.netgear2 [2,342 B]
Get:4 http://apt.readynas.com/packages/readynasos/ 6.2.2/main apache2 armel 2.2.29-0.netgear2 [1,434 B]
Fetched 465 kB in 2s (172 kB/s)
debconf: delaying package configuration, since apt-utils is not installed
Selecting previously unselected package apache2-utils.
(Reading database ... 16315 files and directories currently installed.)
Unpacking apache2-utils (from .../apache2-utils_2.2.29-0.netgear2_armel.deb) ...
Selecting previously unselected package apache2.2-common.
Unpacking apache2.2-common (from .../apache2.2-common_2.2.29-0.netgear2_armel.deb) ...
Selecting previously unselected package apache2-mpm-prefork.
Unpacking apache2-mpm-prefork (from .../apache2-mpm-prefork_2.2.29-0.netgear2_armel.deb) ...
Selecting previously unselected package apache2.
Preparing to replace apache2 2.2.29-0.netgear2 (using .../apache2_2.2.29-0.netgear2_armel.deb) ...
Unpacking replacement apache2 ...
dpkg-maintscript-helper: error: command symlink_to_dir is unknown
Hint: upgrading dpkg to a newer version might help.

Usage: dpkg-maintscript-helper <command> <parameter>... -- <maintainer-script-parameter>...

Commands:
  supports <command>
        Returns 0 (success) if the given command is supported, 1 otherwise.
  rm_conffile <conffile> [<last-version> [<package>]]
        Remove obsolete conffile. Must be called in preinst, postinst and
        postrm.
  mv_conffile <old-conf> <new-conf> [<last-version> [<package>]]
        Rename a conffile. Must be called in preinst, postinst and postrm.
  help
        Display this usage information.
dpkg: warning: subprocess old post-removal script returned error exit status 1
dpkg: trying script from the new package instead ...
dpkg: error processing /var/cache/apt/archives/apache2_2.2.29-0.netgear2_armel.deb (--unpack):
 there is no script in the new version of the package - giving up
Errors were encountered while processing:
 /var/cache/apt/archives/apache2_2.2.29-0.netgear2_armel.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

# apt-get -t testing install dpkg
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
  apache2-mpm-prefork apache2-utils apache2.2-common
Use 'apt-get autoremove' to remove them.
The following extra packages will be installed:
  fontconfig fontconfig-config libfontconfig1 libselinux1
The following packages will be REMOVED:
  apache2
The following packages will be upgraded:
  dpkg fontconfig fontconfig-config libfontconfig1 libselinux1
5 upgraded, 0 newly installed, 1 to remove and 246 not upgraded.
5 not fully installed or removed.
Need to get 0 B/3,926 kB of archives.
After this operation, 684 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
debconf: delaying package configuration, since apt-utils is not installed
(Reading database ... 16776 files and directories currently installed.)
Removing apache2 ...
dpkg-maintscript-helper: error: command symlink_to_dir is unknown
Hint: upgrading dpkg to a newer version might help.

Usage: dpkg-maintscript-helper <command> <parameter>... -- <maintainer-script-parameter>...

Commands:
  supports <command>
        Returns 0 (success) if the given command is supported, 1 otherwise.
  rm_conffile <conffile> [<last-version> [<package>]]
        Remove obsolete conffile. Must be called in preinst, postinst and
        postrm.
  mv_conffile <old-conf> <new-conf> [<last-version> [<package>]]
        Rename a conffile. Must be called in preinst, postinst and postrm.
  help
        Display this usage information.
dpkg: error processing apache2 (--remove):
 subprocess installed post-removal script returned error exit status 1
Errors were encountered while processing:
 apache2
E: Sub-process /usr/bin/dpkg returned an error code (1)

Apache2 isn't recognized :

# service apache2 stop
apache2: unrecognized service

I have backups, and of course I can had reset the NAS. Frontview isn't working anymore, but this is the only trouble, and I would love to be able to fix it without a full reinstall.
The error seems to be well known but I don't understand how to resolve it : https://lists.debian.org/debian-devel/2014/11/msg00108.html

Thank's a lot for your help