Linkify: turning URLs into clickable links in PHP

Turning links like www.example.com and http://twitter.com into clickable links. Sounds like an easy task, right? We’ll there are a few problems that might arise, especially if the text is already HTML formatted.

This function first takes out all potential dangers, by extracting links and tags and replacing them with a placeholder. It than extracts all URLs and replaces them with a placeholder, storing the full HTML link. At the end it replaces all placeholders with the links and tags.

<?php
    /**
     * Turn all URLs in clickable links.
     * 
     * @param string $value
     * @param array  $protocols  http/https, ftp, mail, twitter
     * @param array  $attributes
     * @param string $mode       normal or all
     * @return string
     */
    public function linkify($value, $protocols = array('http', 'mail'), array $attributes = array())
    {
        // Link attributes
        $attr = '';
        foreach ($attributes as $key => $val) {
            $attr = ' ' . $key . '="' . htmlentities($val) . '"';
        }

        $links = array();

        // Extract existing links and tags
        $value = preg_replace_callback('~(<a .*?>.*?</a>|<.*?>)~i', function ($match) use (&$links) { return '<' . array_push($links, $match[1]) . '>'; }, $value);

        // Extract text links for each protocol
        foreach ((array)$protocols as $protocol) {
            switch ($protocol) {
                case 'http':
                case 'https':   $value = preg_replace_callback('~(?:(https?)://([^\s<]+)|(www\.[^\s<]+?\.[^\s<]+))(?<![\.,:])~i', function ($match) use ($protocol, &$links, $attr) { if ($match[1]) $protocol = $match[1]; $link = $match[2] ?: $match[3]; return '<' . array_push($links, "<a $attr href=\"$protocol://$link\">$link</a>") . '>'; }, $value); break;
                case 'mail':    $value = preg_replace_callback('~([^\s<]+?@[^\s<]+?\.[^\s<]+)(?<![\.,:])~', function ($match) use (&$links, $attr) { return '<' . array_push($links, "<a $attr href=\"mailto:{$match[1]}\">{$match[1]}</a>") . '>'; }, $value); break;
                case 'twitter': $value = preg_replace_callback('~(?<!\w)[@#](\w++)~', function ($match) use (&$links, $attr) { return '<' . array_push($links, "<a $attr href=\"https://twitter.com/" . ($match[0][0] == '@' ? '' : 'search/%23') . $match[1]  . "\">{$match[0]}</a>") . '>'; }, $value); break;
                default:        $value = preg_replace_callback('~' . preg_quote($protocol, '~') . '://([^\s<]+?)(?<![\.,:])~i', function ($match) use ($protocol, &$links, $attr) { return '<' . array_push($links, "<a $attr href=\"$protocol://{$match[1]}\">{$match[1]}</a>") . '>'; }, $value); break;
            }
        }

        // Insert all link
        return preg_replace_callback('/<(\d+)>/', function ($match) use (&$links) { return $links[$match[1] - 1]; }, $value);
    }

Do you use Twig? Checkout the Jasny Twig extensions, which includes this as well as many other functions and filters.