Add separate login/password for AdSense spider in password_protect.php. Lets say you added user
adsense123 with password
12345.
You'll have to update settings as follows in your AdSense account:
Restricted Directory or URL: your protected folder or page url.
Authentication URL: same as above.
Authentication method: select
POST.
Parameters: Attribute: access_login
Value: adsense123
Attribute: access_password
Value: 12345
More info about Authentication URL:
http://www.hackszine.com/blog/archive/2007/07/use_adsense_on_password_protec.html