I have been doing a two step glabels-3-batch to LPR process but have made it so I don’t have to write an intermediate file to disk.

Here is my approach in shell scripting

cat merge.csv | glabels-3-batch -i - -o /dev/stdout template.glabels | \
sed -n '/%PDF-1.5/,/%%EOF/p' | lpr -PPDF -J jobname

And in PHP

 /**
     * Send job to gLabels
     *
     * Sends the completed template to the printer held in the $print_settings array
     * @param string $template full path to glabels template
     * @param array $printerDetails Printer Information
     * @return array Array holding the results of the lpr command
     */
    public function glabelsBatchPrint(string $template, $printerDetails)
    {
        $this->setGlabelsTemplate($template);

        $this->setPdfOutFile(); // this is /dev/stdout
        
        // this writes the glabels csv data to disk
        // for testing
        $this->createTempFile($this->printContent);

        $cmdArgs = [
            'glabels-3-batch',
            '-o',
            $this->getPdfOutFile(),
            $this->getGlabelsTemplate(),
        ];

        // when you have csv data your need to merge it into the template
        // from stdin eg. cat merge.csv | glabels-3-batch -i - -o outfile.pdf template.glabels
        if ($this->glabelsMergeCSV) {
            /**
             * add stdin ("-i -") to glabels command line when piping  CSV data into glabels:
             * glabels-3-batch -i - -o
             *      /var/www/wms/app/tmp/20190701182321-customPrint.pdf
             *      /var/www/wms/app/webroot/files/templates/100x50custom.glabels
             */
            array_splice($cmdArgs, 1, 0, ['-i', '-']);
        }
        // printContent is set elsewhere but contains
        // the merge csv data
        $results = $this->runProcess(
            implode(' ', $cmdArgs),
            $this->printContent
        );

        // bug out if the glabel-3-batch process fails
        if ($results['return_value'] !== 0) {
            return $results;
        }
        
        // if successful pull the stdout which was returned from 
        // glabels-3-batch and
        // extract the PDF from the stdout stream
        $pdfPattern = '/(%PDF-1.5.*%%EOF)/s';

        preg_match($pdfPattern, $results['stdout'], $matches);

        /**
         * This grabs the PDF file out of the PDF
         */
        $this->setPrintContent($matches[0]);

        return $this->sendPdfToLpr($printerDetails);
    }

    /**
     * send gLabels PDF to designated LPR printer
     * @param string $printer Print queue name
     * @return array Array with stdout stderr cmd and return_value
     */
    public function sendPdfToLpr($printer)
    {
        $jobId = $this->getJobId();

        $cmdArgs = [
            'lpr',
            '-P',
            $printer,
            '-#',
            $this->glabelPrintCopies,
            '-J',
            $jobId,
            '-T',
            $jobId,
        ];

        $returnValue = $this->runProcess(
            implode(' ', $cmdArgs),
            $this->printContent
        );

        // set back to 0 for next run
        $this->setPrintCopies(0);

        return $returnValue;
    }

    /**
     * run process
     * @param string $cmd Command line to run
     * @param array $printContent Not sure what this is
     * @return array array of stdin, out,err and exit code, cmd
     */
    public function runProcess($cmd, $printContent)
    {
        // code...
        $descriptorspec = [
            0 => ['pipe', 'r'], // stdin
            1 => ['pipe', 'w'], // stdout
            2 => ['pipe', 'w'], // stderr
        ];

        $pipes = [];

        $process = proc_open(
            $cmd,
            $descriptorspec,
            $pipes,
            $this->getCwd(), //cwd orig TMP
            null// env null = current
        );

        // writing straight to stdin works
        fwrite($pipes[0], $printContent);
        fclose($pipes[0]);

        $stdout = stream_get_contents($pipes[1]);
        fclose($pipes[1]);

        $stderr = stream_get_contents($pipes[2]);
        fclose($pipes[2]);

        $return_value = proc_close($process);

        /* return an array with all the necessary information */
        return compact('cmd', 'stdout', 'stderr', 'return_value');
    }