use-posts-on-existing-posts.php

<?php

require 'vendor/autoload.php';

use Symfony\Component\Finder\Finder;

// Find all posts.
$files = Finder::create()->files()->in('../source/_posts')->name('*.md');

foreach ($files as $file) {
    $contents = file_get_contents($file->getPathname());

    // Find the opening front matter tag.
    $position = strpos($contents, '---', 3);

    // Prepend the extra front matter to the second tag.
    $contents = substr_replace($contents, "use: [posts]\n---", $position, 3);

    file_put_contents($file->getPathname(), $contents);
}