You’ll need the PEAR Image_Barcode library for this, which can be installed with the command
#pear install Image_Barcode
Once that is installed, the PHP Code is quite simple. Just use the following:
<?php $number = isset($_GET['number']) ? $_GET['number'] : ''; require_once 'Image/Barcode.php'; header('Content-type: image/png'); Image_Barcode::draw($number, 'code128', 'png'); ?>