rename-post-files.php

<?php

use Symfony\Component\Finder\Finder;

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

$finder = new Finder();
$files = $finder->in(__DIR__.'/source/_posts')->files();

/** @var \Symfony\Component\Finder\SplFileInfo $file */
foreach ($files as $file) {
    $beginsWithDate = preg_match('/^(?<date>\d{4}-\d{2}-\d{2})/', $file->getFilename(), $matches);

    if (!$beginsWithDate) {
        continue;
    }

    // Add the date into the front-matter, after the title.
    $contents = explode("\n", $file->getContents());
    array_splice($contents, 2, 0, "date: {$matches['date']}");
    file_put_contents($file->getPathname(), implode("\n", $contents));

    // Rename the file to remove the date.
    $newPathname = str_replace("{$matches['date']}-", '', $file->getPathname());
    rename($file->getPathname(), $newPathname);
}