URL, Current Page

I want to write a php program that involves recognizing the URL of the page the user is currently on, but I'm not sure exactly how to do this.

What I want from the program is basically this:
If Current URL is ____ then on link click go to this _____ URL.
else if current URL is _____ then on link click go to this ____ URL.
.
.
.
etc.

Is there a way to call the current URL and do this?

Thank in advance!