[horde] Horde + Pecl PAM authentication

[balia]

I want to setup horde webmail 5.2.9 on my server (debian Jessie).
When I go to localhost/horde/test.php I get
  PAM Support (PECL extension): No
  The PAM PECL extension is required to allow PAM authentication to be used.

To install the PAM extension, I have done the following:
# sudo pecl install --alldeps PAM
I can verify that PAM is installed:
# sudo pecl list
Installed packages, channel pecl.php.net:
=========================================
Package   Version State
pam       1.0.3   stable

I have also installed the following:
- libapache2-mod-authnz-pam
- pwauth
- libapache2-mod-authnz-external
I also have the following modules enabled:
  authnz_external, authnz_pam

None of these resolve the error.
What's next?
Thank you.