need help on coding this fuction – redirect with passing in post id

Hi,
I would like to create a redir.php file which will take the post id into and pass in a custom field value based on that post id for redirect . So that the actual url won’t show on the status bar.

It will be something like in the url

http://www.abc.com/redirect.php?id=300

“300” will be the post id and based on that post id, the custom field value “url_field” can be passed in for the redirect link.

Could someone show me how?

thanks