在PHP中合并pdf后，可填写的表单值丢失

I am merging two pdfs using FPDI extended class like this.

function concat() {
    foreach ($this->files AS $file) {
        $page_count = $this->setSourceFile($file);
        for ($i = 1; $i <= $page_count; $i++) {
            $tplidx = $this->ImportPage($i);
            $s = $this->getTemplatesize($tplidx);
            if (strpos($file, 'test') || strpos($file, 'test_slip')) {
                $this->AddPage('L', array($s['w'], $s['h']));
            } else {
                $this->AddPage($this->DefOrientation, array($s['w'], $s['h']));
            }
            $this->useTemplate($tplidx);
        }
    }
}

When I output the merged file, One pdf with fillable field elements had all the elements vanished like the text and barcode etc on that. The other form however is fine. When I echo the fillable pdf form it works great but after this merge I loose all my data from that.

I would appreciate any pointers on this issue.

FPDI does not support importing of form fields.