query regarding pam_set_item

["Varun Vijayvargiya" <varunvijayvargiya gmail com>]

I am an undergraduate currently involved in a project on PAM..actually what i m trying to do is very simple..get a username and password from a remote user accessing my website..use these 2 parameters to authenticate the user using PAM on my server.
The problem:

I can collect the these 2 parameters. But how do i pass it to pam modules so that they understand this.. I mean there is no one at the server who will type the password wen the module prompts for it.

Looking into the code for pam_sm_authenticate calling _unix_read_password() i discovered that before prompting the user for a password, it tries to get it frm pam_get_item().
So i though that before calling pam_authenticate in my application code, i set the incoming password using pam_set_item(handle,PAM_AUTHTOK,"<the passwd>")
But it gives a PAM_BAD_ITEM return value

Am i on the right track or m I losing it. 
Help me out. I m running short of time

Varun