<?php
$url = "Hola, visita http://www.google.com o http://w...content-available-to-author-only...k.com ahora mismo";
echo preg_replace("/\b(https?:\/\/(\w+\.)?\w+\-?\.\w{2,6}(.*?))\b/is", "<a href = '$1'>$1</a>", $url);
PD9waHAKJHVybCA9ICJIb2xhLCB2aXNpdGEgaHR0cDovL3d3dy5nb29nbGUuY29tIG8gaHR0cDovL3cuLi5jb250ZW50LWF2YWlsYWJsZS10by1hdXRob3Itb25seS4uLmsuY29tIGFob3JhIG1pc21vIjsKZWNobyBwcmVnX3JlcGxhY2UoIi9cYihodHRwcz86XC9cLyhcdytcLik/XHcrXC0/XC5cd3syLDZ9KC4qPykpXGIvaXMiLCAiPGEgaHJlZiA9ICckMSc+JDE8L2E+IiwgJHVybCk7