On his Code Diesel blog Sameer takes a look at creating and using barcodes in your PHP applications:

Brought into the mainstream by supermarket checkout systems, bar codes have become a ubiquitous element in our daily lives. Rarely will one come across any product that doesn't have a barcode. The idea of a barcode originated in 1932 from the thesis of Wallace Flint at Harvard.

He touches on a two types of barcodes - standard UPC and the matrix codes (like ShotCode and Semacode) - before looking at how you can create them with the PEAR Image_Barcode package. It supports multiple code types including Code 39, EAN 13, INT 25 and PostNet. Output examples are included.