Thanks for your suggestion. I solved it as follows. It may be more steps than I need, but at my level it' great that it works.
$pattern = "(http://)|(www).+";
$desc_text = $row_data->ldesc;
$array = split(' ', $row_data->ldesc);
for($i=0; $i < count($array); $i++){
if (eregi($pattern, $array[$i])){
$desc_url = "<A HREF=\"$array[$i]\">$array[$i]</A>";
$desc_text = eregi_replace($array[$i], $desc_url, $desc_text);
}//end if
} //end for
print "<DD>$desc_text<BR><BR>";