PHP script to parse markdown for headers

md-to-html-header-markup.php

<?php
/*
Let's write a simple markdown parser function that will take in a single line of markdown and be translated into the appropriate HTML. To keep it simple, we'll support only one feature of markdown in atx syntax: headers.

Headers are designated by (1-6) hashes followed by a space, followed by text. The number of hashes determines the header level of the HTML output.

Examples

# Header will become <h1>Header</h1>
## Header will become <h2>Header</h2>
###### Header will become <h6>Header</h6>
*/


/**
 * Header markdown to HTML markup
 *
 * @param string $markdown
 * @return string
 */
function markdown_parser($markdown)
{
    $markup = preg_replace_callback(
        '/^(#+)(.+?)\n/',
        function ($matches) use ($markdown) {
            // bail if regex does not catches anything
            if ($matches[1] == "") {
                return trim($markdown);
            }

            // if there is no space after the markdown "#"s reject it.
            if (!preg_match('/\s/', $matches[2])) {
                return "#" . $matches[2];
            }

            $hash_count = strlen($matches[1]);
            // Bail if hash count is not in range of 1-6.
            if (! in_array($hash_count, range(1, 6), true)) {
                return trim($markdown);
            }

            return html_entity_decode("<h$hash_count>" . trim($matches[2]) . "</h$hash_count>");
        },
        // Adding "\n" to fit our regex.
        ltrim($markdown) . "\n"
    );
    // final cleanup
    return trim(preg_replace('~[\r\n]+~', ' ', $markup));
}

echo markdown_parser('Behind # The Scenes');
echo PHP_EOL;
echo markdown_parser('# The Scenes');
echo PHP_EOL;
echo markdown_parser("#invalid");
echo PHP_EOL;
echo markdown_parser("##invalid");
echo PHP_EOL;
echo markdown_parser("## header");
echo PHP_EOL;
echo markdown_parser("### header");
echo PHP_EOL;
echo markdown_parser("#### header");
echo PHP_EOL;
echo markdown_parser("## The Scenes");