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function plain_url_to_link($string) {
  return preg_replace(
  '%(https?|ftp)://([-A-Z0-9./_*?&;=#]+)%i', 
  '<a target="blank" rel="nofollow" href="$0" target="_blank">$0</a>', $string);
}

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$str = "http://www.google.com is a search website, you can also visit http://yahoo.com and http://twitter.com";
echo plain_url_to_link($str);