While you could hypothetically send the raw ZPL / EPL commands to the printer device, you might be better off not doing that if you don't already know ZPL / EPL, and if you can already generate images in your environment.
Your code implies that you're on a Unix-like system. If you're on a recent Unix-like system, then printing should be controlled by CUPS. Zebra has published unsupported but mostly functional CUPS support files.
Set up the printer in CUPS, then shell out to /usr/bin/lp with the -d flag set to the printer name, and a -o ppi=... value to set the DPI of the printer, and possibly other things to force alignment or portrait/landscape mode. That GK420s is a 203 DPI printer, so you'd want -o ppi=203 at the very minimum.
You can then print anything to that printer that CUPS can understand, including images and PDF documents. This allows you to composite anything you want PHP-side, not restricting you to command language that the printer understands. For example, we use wkhtmltoimage to build shipping labels, while we use GD and PEAR's prehistoric Image_Barcode to produce small barcode labels. There are better options for that, by the way.
Alternatively, you can set up a "Generic Raw" virtual printer in CUPS. You can then print command language text files directly through that printer. You should probably only do it that way if you are comfortable and familiar with EPL or ZPL.
The following code is a cut down section of real, live code that we use to print to all of our printers, including Zebras. Just call the function below with the data you want printed (such as the contents of an image, or text, or whatever) as the first argument.
function print_example($data) {
// You'll need to change these according to your local names and options.
$server = 'printserver.companyname.com';
$printer_name = 'Zebra_ZP_500_KB'; // That's effectively the same thing as your GK420d
$options_flag = '-o position=bottom-left,ppi=203,landscape';
$process_name = 'LC_ALL=en_US.UTF-8 /usr/bin/lp -h %s -d %s %s';
$command = sprintf($process_name, $server, $printer_name, (string)$options_flag);
$pipes = array();
$process = proc_open($command, $handles, $pipes);
// Couldn't open the pipe -- shouldn't happen
if (!is_resource($process))
trigger_error('Printing failed, proc_open() did not return a valid resource handle', E_USER_FATAL);
// $pipes now looks like this:
// 0 => writeable handle connected to child stdin
// As we've been given data to write directly, let's kinda like do that.
fwrite($pipes[0], $data);
fclose($pipes[0]);
// 1 => readable handle connected to child stdout
$stdout = fgets($pipes[1]);
fclose($pipes[1]);
// 2 => readable handle connected to child stderr
$stderr = fgets($pipes[2]);
fclose($pipes[2]);
// It is important that you close any pipes before calling
// proc_close in order to avoid a deadlock
$return_value = proc_close($process);
// We've asked lp not to be quiet about submitting jobs so we can make
// sure that the print job was submitted.
$request_match = array();
if (!preg_match('/request id is\b(.+)/', $stdout, $request_match)) {
add_warning("Print to '$printer' failed. Please check the printer status.");
return false;
}
add_notice("Print to '$printer' succeeded. Job $request_match[1].");
return true;
}
The functions add_warning and add_notice are implemented in our code, you'll need to replace them as appropriate for whatever you're actually printing.
