autolinkHashtags.php

<?php

/**
 *  Given a string and a baseurl, finds all hashtags matching
 * `#[\-_a-zA-Z0-9]+` and wraps them in an `a` element with `rel=tag` set
 * and a `href` of baseurl + '/' + tagname without the #.
 */
function autolinkHashtags($text, $baseUrl) {
	$baseUrl = rtrim($baseUrl, '/');

	// $replacements = ["#tag" => "<a rel="tag" href="/tags/tag">#tag</a>]
	$replacements = array();
	$matches = array();

	if (preg_match_all('/(?<=^|\s)\#([a-zA-Z0-9\-\_]+)/i', $text, $matches, PREG_PATTERN_ORDER)) {
		// Look up #tags, get Full name and URL
		foreach ($matches[0] as $name) {
			$name = str_replace('#', '', $name);
			$replacements[$name] = '<a rel="tag" href="' . $baseUrl . '/' . $name . '">#' . $name . '</a>';
		}

		// Replace #tags with valid microformat-enabled link
		foreach ($replacements as $name => $replacement) {
			$text = str_replace('#' . $name, $replacement, $text);
		}
	}

	return $text;
}