Extends [Parsedown](https://github.com/erusev/parsedown) with checkboxes and (closing) tags replacement.

Parsedown.php

<?php

use Parsedown as ParsedownBase;

/**
 * Extends [Parsedown](https://github.com/erusev/parsedown) with :
 * - Replacement of [ ] by checkbox
 * - Replacement of tags by other (only closing tags).
 * Usage :
 *       $Parsedown = new Parsedown();
 *       $Parsedown->supportCheckboxes();
 *       $Parsedown->setReplaceTags([
 *          'h3' => 'h4',
 *          'h2' => 'h3',
 *          'h1' => 'h2',
 *      ]);
 *      echo $Parsedown->text('CHANGELOG.md');
 */
class Parsedown extends ParsedownBase
{

	protected $checkboxes = false;
	protected $replace_tags = [];

	/**
	 * Set that we want to replace [ ] with checkboxes
	 * 
	 * @param bool $flag
	 */
	public function supportCheckboxes(bool $flag = true)
  {
		$this->checkboxes = $flag;
	}

	/**
	 * Set tags to replace
	 * 
	 * @param array $tags
	 */
	public function setReplaceTags(array $tags = [])
  {
		$this->replace_tags = $tags;
	}

	/**
	 * Override for custom modifications
	 * 
	 * @param string $text
	 * @return string
	 */
	public function text($text)
	{
		$markup = parent::text($text);

		if($this->replace_tags) {
			$markup = $this->replaceTags($markup);
		}

		if($this->checkboxes) {
			$markup = $this->replaceCheckboxes($markup);
		}

		return $markup;
	}

	/**
	 * Replace tags by other.
	 * Warning : only with closing tags (titles, link, etc...), not inline tags (like br, hr...)
	 * 
	 * @param string $markup
	 * @return string
	 */
	protected function replaceTags(string $markup): string
	{	
		$open_before = array_map(function($v) {
			return "<$v>";
		}, array_keys($this->replace_tags));
		$open_after = array_map(function($v) {
			return "<$v>";
		}, array_values($this->replace_tags));

		$close_before = array_map(function($v) {
			return "</$v>";
		}, array_keys($this->replace_tags));
		$close_after = array_map(function($v) {
			return "</$v>";
		}, array_values($this->replace_tags));

		$markup = str_replace($open_before, $open_after, $markup);
		$markup = str_replace($close_before, $close_after, $markup);

		return $markup;
	}


	/**
	 * Replace checkboxes
	 * 
	 * @param string $markup
	 * @return string
	 */
	public function replaceCheckboxes(string $markup): string
	{
		$markup = str_replace('[ ]', '<input type="checkbox" disabled>', $markup);
		$markup = str_replace('[x]', '<input type="checkbox" disabled checked>', $markup);

		return $markup;
	}
}