How to Use Google API to Find PR?

Is this possible and leagal?

[webgaya]

I have seen that certain sites and software
show Google PR for a given URL.

How do they do this? Are they using any
particular Google API?

Is it needed a special permission from
Google to do this?

Can any server side scripting language
(Perl or PHP) help me to build a script
that shows the Google PR?

If it is possible, which data do I need
from Google?