Código PHP:
function links($cadena,$target="")
{
$http = "http://";
$cadena = str_replace('<', ' <', $cadena);
$cadena = str_replace('>', '> ', $cadena);
$array = split(" ", $cadena);
$targt = ($target) ? " target=\"".$target."\"":"";
for($i=0;$i<=(count($array));$i++)
{
if(eregi($http, $array[$i]))
$copia[$i] = "<a href=\"".$array[$i]."\"$targt>$array[$i]</a>";
else
$copia[$i] = $array[$i];
}
return implode(' ', $copia);
}
$cadena = "Este es un link http://www.comoandas.com<br>y este es otro link http://www.google.com.";
echo "La cadena original es: <br>$cadena<br>";
//$links = links($cadena);
//le agregara.. target="_blank"
$links = links($cadena,"_blank");
echo $links;
Listo :P