Simple USPS tracking link generator in PHP

On March 26, 2017, the USPS tracking page stopped working for me. Argh. It seems like the people who make the web pages can't stop changing things. For... well, forever, it worked fine. Now it doesn't. Every time I enter a tracking number, it goes back to the same starting screen.

Since I am getting started with PHP, why not whip up a simple script to create a working link to save me the trouble of doing it with cut and paste? I hope at least one person finds it helpful in some way.