Merge PDF files with custom titles using Ghostscript

merge_pdf.php

<?php

// merge_pdf_files(['file1.pdf', 'file2.pdf', 'file3.pdf'], ["title a", "title b", "title c"], "outfile.pdf")

function encode_ps_base($str) { // <hex encoded UTF-16BE with BOM>
    return '<'.bin2hex($str).'>';
}
function encode_ps_utf16($str) { // <hex encoded UTF-16BE with BOM>
    $utf16be = "\xFE\xFF".mb_convert_encoding($str, 'UTF-16BE', 'UTF-8');
    return encode_ps_base($utf16be);
}

function merge_pdf_files($files, $titles, $outfile) {
    $cmd = 'gs -q -dNOSAFER -sDEVICE=pdfwrite -o '.escapeshellarg($outfile).' - 2>&1';

    $pipes = [];
    $proc = proc_open($cmd, [
        0 => ['pipe', 'r'],
        1 => ['pipe', 'w'],
    ], $pipes);

    if (!is_resource($proc)) {
        return false;
    }

    $encoded_files = implode(' ', array_map('encode_ps_base', $files));
    $encoded_titles = implode(' ', array_map('encode_ps_utf16', $titles));
    
    $script = <<<SCRIPT
    /files [{$encoded_files}] def
    /titles [{$encoded_titles}] def
    
    /fileCount files length def
    
    0 1 fileCount 1 sub {
      /index exch def
      files index get /fileName exch def
      titles index get /title exch def
    
      /pagesDone currentpagedevice /PageCount get 1 add def
 
      fileName run [ /Page pagesDone /Title title /OUT pdfmark
    } for

SCRIPT;

    fwrite($pipes[0], $script);
    fclose($pipes[0]);
        
    $err = stream_get_contents($pipes[1]);
    fclose($pipes[1]);

    $exit_code = proc_close($proc);

    if ($exit_code) {
        trigger_error("Ghostscript error: $err", E_USER_WARNING);
    }

    return $exit_code === 0;
}

mergepdf.ps

% gs -dNOSAFER -sDEVICE=pdfwrite -o finished.pdf mergepdf.ps

% replace these
/files [(file1.pdf) (file2.pdf) (file3.pdf)] def

% might want to use hex encoding, but then it must be UTF-16BE with BOM
/titles [(title1) (title2) (title3)] def

/fileCount files length def

0 1 fileCount 1 sub {
  /index exch def
  files index get /fileName exch def
  titles index get /title exch def

  /pagesDone currentpagedevice /PageCount get 1 add def
  
  % "[" is a ... mark operator, so it doesn't necessarily start the array
  % so it's like the "pdfmark" pops it, maybe "]pdfmark" 'd make it more clear
  
  fileName run [ /Page pagesDone /Title title /OUT pdfmark
} for