removing parts of a string...

I wanna be able to take a URL http://www.mysite.com/downloads/file_1_a.pdf and then remove: http;//www.mysite.com/ then add: /host/sites/site1/web/ to downloads/file_1_a.pdf Turning: http://www.mysite.com/downloads/file_1_a.pdf Into: /host/sites/site1/web/downloads/file_1_a.pdf I know I need to look up strings, but can someone point me in the right direction (I wanna learn, so don't give too much away!) ********************************************************************* The information contained in this e-mail message is intended only for the personal and confidential use of the recipient(s) named above. If the reader of this message is not the intended recipient or an agent responsible for delivering it to the intended recipient, you are hereby notified that you have received this document in error and that any review, dissemination, distribution, or copying of this message is strictly prohibited. If you have received this communication in error, please notify us immediately by e-mail, and delete the original message. ***********************************************************************