I haven't used printing with PHP, but try this
$printer = "printername");
if($ph = printer_open($printer))
{
$fh = fopen("filename.ext", "rb");
$content = fread($fh, filesize("filename.ext"));
fclose($fh);
printer_set_option($ph, PRINTER_MODE, "RAW");
printer_write($ph, $content);
printer_close($ph);
}
else "Couldn't connect...";