There is a library for this BarCode PHP. You just need to include a few files:
require_once('class/BCGFontFile.php');
require_once('class/BCGColor.php');
require_once('class/BCGDrawing.php');
You can generate many types of barcodes, namely 1D or 2D. Add the required library:
require_once('class/BCGcode39.barcode.php');
Generate the colours:
// The arguments are R, G, and B for color.
$colorFront = new BCGColor(0, 0, 0);
$colorBack = new BCGColor(255, 255, 255);
After you have added all the codes, you will get this way:
Example
Since several have asked for an example here is what I was able to do to get it done
require_once('class/BCGFontFile.php');
require_once('class/BCGColor.php');
require_once('class/BCGDrawing.php');
require_once('class/BCGcode128.barcode.php');
header('Content-Type: image/png');
$color_white = new BCGColor(255, 255, 255);
$code = new BCGcode128();
$code->parse('HELLO');
$drawing = new BCGDrawing('', $color_white);
$drawing->setBarcode($code);
$drawing->draw();
$drawing->finish(BCGDrawing::IMG_FORMAT_PNG);
If you want to actually create the image file so you can save it then change
$drawing = new BCGDrawing('', $color_white);
to
$drawing = new BCGDrawing('image.png', $color_white);