Sure! You can get PHP to call any external program you want, using (read up on the manual to see which is most appropriate for you) exec(), system(), or passthru(). On linux, the print utility is called "lpr", so
system("lpr $filename");
will do the job. A warning: if $filename is user-supplied, you will want to make sure to use $filename = escapeshellcmd($filename) before you allow $filename to appear in any system, exec, or passthru command; otherwise, you have a potential security hole. Read up on these functions in the PHP manual for more info.
HTH,
AC