I'd use a regular expression and capture the text. Depending on how serious you are about getting actual URL-friendly characters, something as simple as (heh):

# $text contains the text to be 'urled'
$text=~ s/(www\.\S+)/<a href="http:\/\/$1">$a<\/a>/g;
[download]

might do you just fine. (explanation: looks for "www" followed by a period and one or more non-whitespace characters. remembers all of that. $1 is where the memorized value goes. The rest is left as an exercise for the reader).

search keywords: regular, expression, memory

Philosophy can be made out of anything. Or less -- Jerry A. Fodor