Remove forced `/blog/` prefix added to main site permastruct in Wordpress multisite environments

fix-wpmu-permastruct.php

<?php

global $WPMUPermastructFix;
$WPMUPermastructFix = new MultisitePermastructFix();

class MultisitePermastructFix
{
	public function __construct()
	{
		// start the checking process for main multisite blog. Will strip forced '/blog/' from the start.
		add_action('generate_rewrite_rules', array($this, 'checkBlogPrefix'), PHP_INT_MAX);
	}

	// if the environment is what we're looking for, we will reprocess and force permalinks after Wordpress has overridden everything.
	public function checkBlogPrefix($wp_rewrite)
	{
		if (is_multisite() && !is_subdomain_install() && is_main_site()) {
			add_action('shutdown', array($this, 'stripBlogPrefix'));
		}
	}

	// strip prefix from all permastruct data
	public function stripBlogPrefix()
	{
		global $wp_rewrite;

		$searchRegex = '@^/?blog/@';

		// strip from main permalink structures
		$wp_rewrite->set_permalink_structure(preg_replace($searchRegex, '', $wp_rewrite->permalink_structure));
		$wp_rewrite->set_category_base(preg_replace($searchRegex, '', $wp_rewrite->get_category_permastruct()));
		$wp_rewrite->set_tag_base(preg_replace($searchRegex, '', $wp_rewrite->get_tag_permastruct()));

		// reprocess entire rules arrays in case any custom ones are missed. Category/tag base also doesn't seem fully updated via above.
		$wp_rewrite->rules = $this->stripBlogPrefixArr($wp_rewrite->rules, $searchRegex);
		$wp_rewrite->extra_rules = $this->stripBlogPrefixArr($wp_rewrite->extra_rules, $searchRegex);
		$wp_rewrite->extra_rules_top = $this->stripBlogPrefixArr($wp_rewrite->extra_rules_top, $searchRegex);

		// also reprocess custom permastructs since the first step stripping doesn't actually change them
		foreach ($wp_rewrite->extra_permastructs as $structName => $struct) {
			if ( is_array( $struct ) ) {
				if ( count( $struct ) == 2 ) {
					$wp_rewrite->extra_permastructs[$structName][0] = preg_replace($searchRegex, '', $struct[0]);
				} else {
					$wp_rewrite->extra_permastructs[$structName]['struct'] = preg_replace($searchRegex, '', $struct['struct']);
				}
			} else {
				$wp_rewrite->extra_permastructs[$structName] = preg_replace($searchRegex, '', $struct);
			}
		}

		// force an update
		$wp_rewrite->flush_rules();
	}

	protected function stripBlogPrefixArr($rules, $searchRegex)
	{
		$newRules = $rules;

		foreach ($rules as $k => $v) {
			$newk = preg_replace($searchRegex, '', $k);
			if ($newk != $k) {
				$newRules = $this->replaceArrayKey($newRules, $k, $newk);
			}
		}

		return $newRules;
	}

	protected function replaceArrayKey($array, $old_key, $new_key)
	{
		$keys = array_keys($array);
		if (false === $index = array_search($old_key, $keys)) {
			return $array;
		}
		$keys[$index] = $new_key;
		return array_combine($keys, array_values($array));
	}
}