Function to convert any URLs in a given string to links.

url_to_link.php

<?php
/**
 * Converts any URLs in a given string to links.
 *
 * @param string $str The string to be searched for URLs to convert to links.
 * @return string
 **/
function url_to_link($str)
{
    $pattern = "((https?://)+[\w\d:#@%!/;$()~_?\+-=\\\.&]*)";
    $str = preg_replace($pattern, '<a href="\0">\0</a>', $str);
    return $str;
}