mpdf Stich PDFS or Merge multiple pdfs

mPDF.php

<?php
function custom_yith_merge_PDF_Slips(Array $filenames, $outFile) {
  
    $mpdf = new mPDF();
    if ($filenames) {
        $filesTotal = sizeof($filenames);
        $fileNumber = 1;
        $mpdf->SetImportUse();
        if (!file_exists($outFile)) {
            $handle = fopen($outFile, 'w');
            fclose($handle);
        }
        foreach ($filenames as $fileName) {
            if (file_exists($fileName)) {
                $pagesInFile = $mpdf->SetSourceFile($fileName);
                //print_r($fileName); die;
                for ($i = 1; $i <= $pagesInFile; $i++) {
                    $tplId = $mpdf->ImportPage($i);
                    $mpdf->UseTemplate($tplId);
                    if (($fileNumber < $filesTotal) || ($i != $pagesInFile)) {
                        $mpdf->WriteHTML('<pagebreak />');
                    }
                }
            }
            $fileNumber++;
        }
        $mpdf->Output();
    }
}