Google Chart API has a QR Code barcode generator (no additional software installed on your server)
https://developers.google.com/chart/infographics/docs/qr_codes(https://developers.google.com/chart/infographics/docs/qr_codes)
Or alternatively you could utilize GD to generate standard bar codes. This appears to be a decent tutorial:
http://davidscotttufts.com/2009/03/31/how-to-create-barcodes-in-php(http://davidscotttufts.com/2009/03/31/how-to-create-barcodes-in-php)
If you prefer video tutorials this might help.
http://www.youtube.com/playlist?list=PL0AB5FA7DEF91AB58(http://www.youtube.com/playlist?list=PL0AB5FA7DEF91AB58)