have a look at the man page for [man]ereg_replace[/man] - i've taken the example from there and adapted it below
$text = ereg_replace("[[:alpha:]]+://[^<>[:space:]]+[[:alnum:]/]", "<a href=\"\\0\"><font color=\"red\">\\0</font></a>", $text);
this should take all urls and put them in an A tag as well as a put the FONT tag around it
note: if your links are already in the form <A HREF="...">...</A> it will mess them up, as it does the whole adding of <A also
adam