Convert (civicrm-docs) markdown to epub

convert.php

#!/usr/bin/env php
<?php

// composer require symfony/yaml (or incorperate it into civicrm-docs (or similar) to get this up and running.

require __DIR__ . '/vendor/autoload.php';

if(!realpath($argv[1])){
  die("Could not find {$argv[1]}\n");
}

$mkdocsFile = realpath($argv[1]);
$mkdocsDir = dirname($mkdocsFile);


$mkdocs = Symfony\Component\Yaml\Yaml::parse(file_get_contents($mkdocsFile));

$flattened = [];
flatten($mkdocs['pages'], $flattened);

$epubMetadata = [
  'identifier' => $mkdocs['repo_url'],
  'title' => $mkdocs['site_name'],
  'creator' => $mkdocs['site_author'],
  'date' => date('YYYY-MM-DD'),
  'lang' => '', // TODO
  'description' => $mkdocs['site_description'],
  'rights' => '' // TODO we should require licenses on all books we publish
];

$titleFile = '{$mkdocsDir}/docs/.civicrm-docs-epub-metadata';
file_put_contents($titleFile, Symfony\Component\Yaml\Yaml::dump($epubMetadata));
$outputFile = preg_replace('/\W+/', '-', strtolower($mkdocs['site_name'])) . '.epub' ;

foreach($flattened as $chapter){
  if($chapter['file']){
    $chapters[] = "{$mkdocsDir}/docs/{$chapter['file']}";
  }
}

$chapters = implode(' ', $chapters);

// Unfortunatley, I hit a roadblock with how pandoc handles relative URLs. When creating the epub, it produces lots of 
// errors along the lines of `pandoc: Could not find media `../img/petition_new.png', skipping...`
// Basically, pandoc concats all the chapters and presumes that the URLs are relative to the first chapter that you added.
// This assumption is not valid when you have documentation in subdirectories (like the user and developer guide do).
//
// One solution is to use https://github.com/jgrassler/mkdocs-pandoc (which I discovered when I hit this roadblock
// and might make a lot of this script obselete. I coudn't get that lib to work though didn't spend too long trying.
//
// Another would be to do our own pre-processesing of the markdown or make a copy of the images where mkdocs would be able
// to find them. I did try and do that by symlinking everthing in docs/img to img in the hope that the ../ would then resolve
// but it didn't.
//
// So not an insurmountable problem, but something for another day...

exec("pandoc -S -o $outputFile $titleFile $chapters");

unlink($titleFile);

// Functions

function flatten($section, &$flattened){
  foreach($section as $chapter){
    foreach($chapter as $title => $file){
      if(is_array($file)){
        $flattened[]=['title' => $title, 'file' => ''];
        flatten($file, $flattened);
      }else{
        $flattened[]=['title' => $title, 'file' => $file];
      }
    }
  }
}