// Returns the text given as parameter, with contained URLs interpreted as HTML links

function link($text){
  return preg_replace('@((https?://)?([-\w]+\.[-\w\.]+)+\w(:\d+)?(/([-\w/_\.]*(\?\S+)?)?)*)@', '<a href="$1">$1</a>', $text);
}

A source to help you interpreting URLs in plain text as a HTML link. It's an improvement of what was proposed here : http://snipplr.com/view/2371/regex-regular-expression-to-match-a-url/

Improvement of URL interpretation with regex

regex, url, php

PHP