If you are using the stand alone version (not the one installed on blogs), make sure that you copy and paste all of the code into the very top of the php page (above the <hhtml> tag).
Be sure to save the page as a php page (make sure that your server can run php and isn't windows based - unless it's specifically a windows server you're probably fine).
So now,you should have your page named "whatever.php"
Now, in the code that you just placed above the html tags, there is a line that should say:
$cpa_offer_url =
now inside of the single quotes (the quotes that are ' ' ) put in the url of wherever you want to redirect to (so put in google if you want).
Now just above that, is your magic number area - you can change it to whatever you want or leave it as 1234 if you want.
Save and upload this file, and then call it as you said (somesite.com/whatever.php?mn=1234)
It should redirect like a charm
Will this affect pagerank: no
You are only using this url to hide your referrer - not for seo, and because this is done in php (which is a server side language) google doesn't see the code.
Because of that, your page rank shouldn't change, as they would be referenced as "whatever.php" not "whatever.php?mn=1234
Hope this helps