Pdmain is used to login to publicdata services.
Login is only allowed using POST or GET method. Please log in Once a day only. Following is an example POST link.
https://login.publicdata.com/pdmain.php/logon/checkAccess?disp=XML
 
Lets take a look at the link for PublicData's logon service.
https://login.publicdata.com
 - This is the main server to login.  After successful authentication, it will give you the login id for one day.
 
pdmain.php
 - since we are performing a logon to the publicdata system we call pdmain.php
 
/Logon/checkAccess
 - this is the logon service that will run when called.
 
disp=XML
 - this is the display format that we wish to receive. Since we are an API user this will be XML.
 
Note: We use only use POST for login requests because this is more secure.
Lets take a look at the variables you will need to pass in.
login_id='SDF000006'
 - this is same username that you logged in with. 
 
password='nottellingyou'
 - this is the same password that you logged in with normally.
 
Here is the response from that request.
Now Lets break down the XML level by level.
termsconditions_agreement
Description: 
These are the terms and conditions you agree to whenever you use PublicData's service. The hyperlink contains all details of the contract you agree to.
Path: 
pddata->termsconditions_agreement
 
postlogondata
Description: 
This node is meant to tell you how many look-ups you have left.  Use this formula to find out the look-ups remaining: [(searchallowed + searchbonus) - searchused].
Path: 
pddata->postlogondata
 
helplinks
Description: 
Any other helpful links that could help you.
Path: 
pddata->helplinks
 
user
Description: 
This node holds the authorization id if you successfully log in.
Path: 
pddata->user