Cannot get PHP PAM authentication to work

This question is split between the realm of SO and SF. It's for a coding project but I believe the error I'm encountering has more to do with system administration than code, so I'll put it here.

I'm trying to make a PHP login page that authenticates users on their pre-existing system user accounts. I installed the php5 pam authentication module and followed the documentation to configure it.

I created /etc/pam.d/php with the following contents:

# /etc/pam.d/php
#
# note: both an auth and account entry are required

auth    sufficient      /lib/security/pam_pwdb.so shadow nodelay
account sufficient      /lib/security/pam_pwdb.so

I added "pam.servicename = "php";" to my /etc/php5/apache2/php.ini file and then restarted my apache service.

Every time my script uses the pam_auth function, the following appears in my auth.log:

Jan  6 08:56:33 cobalt apache2: PAM unable to dlopen(/lib/security/pam_pwdb.so)
Jan  6 08:56:33 cobalt apache2: PAM [error: /lib/security/pam_pwdb.so: cannot open shared object file: No such file or directory]
Jan  6 08:56:33 cobalt apache2: PAM adding faulty module: /lib/security/pam_pwdb.so

How can I fix this? For something as simple (or so I thought), should I just write my own functions to parse /etc/shadow and /etc/passwd? I'm not trying to log the user into the system itself, I just need to let the user log into my script with their system user/password and check that they belong to an appropriate group to run a given report.