In order to invoke screen-scraper from PHP, screen-scraper needs to be running in server mode. If you'd like a refresher on how to start up screen-scraper in server mode go ahead and follow that link, then come back here.
Your PHP code will need to refer to screen-scraper's PHP driver, called remote_scraping_session.php. You can find this file in the misc\php\ folder of your screen-scraper installation. You'll want to copy the file into the directory where you plan on putting the PHP file that will invoke screen-scraper.
Download the shopping.php file and then save it in the same directory where you copied the remote_scraping_session.php file. Rename the file from shopping.php.txt to shopping.php.
Open up your web browser and go to the URL corresponding to the shopping.php file (e.g, "http://localhost/screen-scraper/shopping.php"). You'll see a simple search form. Type in a product keyword, such as bug, then hit the Go button. If all goes well the page will take a little while to load (it's waiting as screen-scraper extracts the data), then it will output the corresponding products.
If that didn't go quite as you expected here are some things to check:
Assuming that test worked, fire up your favorite PHP editor and open the shopping.php file in it. The file is pretty heavily commented, so hopefully it makes sense what's going on. If not, try reviewing the PHP documentation or posting to our forum.
When you invoke screen-scraper as a server it creates log files corresponding to your scraping session in its log folder. Take a look in that folder for your Shopping Site log file and take a look through it. It should look similar to what you see when you run scraping sessions in the workbench.