Link between planned order and purchase requisition

Hi,

What is the link between planned order and purchase requisition? If I have the number of a planned order that has already been converted to a PR, but I don't know the PR number, how can I find out what the PR is? I also need to know the opposite, i.e. if I have a PR that has been created from a planned order, how do I determine which planned order it was created from? Help is appreciated, thanks.

Regards