PHP Markdown Filter

Motivation

As Markdown has grown in popularity, many people have expected to see new formatting syntaxes added. However, people who develop Markdown parsers will generally stick to the philosophy that John Gruber has explained, that the design goal of Markdown’s formatting syntax is to make it as readable as possible. The idea is that a Markdown-formatted document should be publishable as-is, as plain text, without looking like it’s been marked up with tags or formatting instructions.

Typically, they will simply tell people to use raw HTML syntax if their wishes are too complex and/or not in line with Markdown’s philosophy. Markdown parser generally does not prohibit people from doing so.

People who just know how to search and replace text with PHP often give naive suggestions, such as telling people to use regular expressions to replace text directly in the Markdown document, which often ends up with people replacing text in the wrong places, such as replacing text inside a code block syntax that should be left as it is.

This filter can be used to separate parts of a Markdown document into blocks and spans, so that you can replace text only in certain blocks and spans that you consider safe.

Usage

This converter can be installed using Composer, but it doesn’t need any other dependencies and just uses Composer’s ability to automatically include files. Those of you who don’t use Composer should be able to include the index.php file directly into your application without any problems.

Using Composer

From the command line interface, navigate to your project folder then run this command:

composer require taufik-nurrohman/markdown-filter

Require the generated auto-loader file in your application:

<?php

use function x\markdown_filter\row as filter_row;
use function x\markdown_filter\rows as filter_rows;

require 'vendor/autoload.php';

$content = file_get_contents('.\path\to\file.md');

$content = filter_rows($content, function ($block, $status) {
    if (0 === $status || 2 === $status) {
        return $block;
    }
    return filter_row($block, function ($chop, $status) {
        if (0 === $status) {
            return $chop;
        }
        // Safely replace `:)` with `😊`
        return strtr($chop, [':)' => '&#128522;']);
    });
});

// You can now convert the Markdown document to HTML using your preferred Markdown converter
echo (new ParsedownExtra)->text($content);

Using File

Require the index.php file in your application:

<?php

use function x\markdown_filter\row as filter_row;
use function x\markdown_filter\rows as filter_rows;

require 'index.php';

$content = file_get_contents('.\path\to\file.md');

$content = filter_rows($content, function ($block, $status) {
    if (0 === $status || 2 === $status) {
        return $block;
    }
    return filter_row($block, function ($chop, $status) {
        if (0 === $status) {
            return $chop;
        }
        // Safely replace `:)` with `😊`
        return strtr($chop, [':)' => '&#128522;']);
    });
});

// You can now convert the Markdown document to HTML using your preferred Markdown converter
echo (new ParsedownExtra)->text($content);

The $status variable shows whether a part of the document is safe or not for any kind of text substitutions. For now, it can have the value set to 0, 1, or 2. A value of 0 means that the part of the document is generally not safe for any kind of text substitutions. It is typically contained in the code and raw HTML chunks. A value of 2 means that a block can contain other blocks, so it would be better to skip it as well, because indentation usually has a different meaning in this situation, until then this filter reaches into the inner content of that block.

The main goal of this project is to introduce the “embed” syntax for Markdown, which I believe has never been discussed before (for this kind of syntax). That’s why I implemented this filter on the test page as a sort of utility to safely replace the syntax:

You can also use this filter to strip HTML tags other than those that are written in Markdown’s code syntax. People usually write HTML syntax there to share a piece of code in your comments section:

<?php

use function x\markdown_filter\row as filter_row;
use function x\markdown_filter\rows as filter_rows;

if ('POST' === $_SERVER['REQUEST_METHOD'] && isset($_POST['comment']['content'])) {
    $_POST['comment']['content'] = filter_rows($_POST['comment']['content'], function ($block, $status) {
        if (2 === $status) {
            return $block;
        }
        if (0 === $status) {
            $dent = strspn($block, ' ');
            if ($dent >= 4) {
                return $block; // Code block (indent-style)
            }
            $test = substr($block, $dent);
            if (0 === strpos($test, '```') || 0 === strpos($test, '~~~')) {
                return $block; // Code block (fence-style)
            }
            return strip_tags($block);
        }
        return filter_row($block, function ($chop, $status) {
            if (0 === $status) {
                $test = strspn($chop, '`');
                if ($test > 0 && str_repeat('`', $test) === substr($chop, -$test)) {
                    return $chop; // Code span
                }
            }
            return strip_tags($chop);
        });
    });
}

Tests

Clone this repository into the root of your web server that supports PHP and then you can open the test.php file with your browser.

License

This library is licensed under the MIT License. Please consider donating 💰 if you benefit financially from this library.