I could not find any function to replace an url within a string with a link

if someone writes http://www.php.net, I want I to be replaced with <a href="http://www.php.net>

I saw this on your page too, its easy to recreate your function myself with 

$string = preg_replace("((http://)([a-zA-Z1-9\-_\.]+\.[a-zA-Z1-9\-_]+\.[a-z]{2,3}(/[a-z\-/\._\?=&]+)?))", "<a href=\"\\1\\2\" target=\"_blank\">\\0</a>", $string);

but wouldnt it be nice to replace an url like www.php.net too?

For now, I use

$string = preg_replace("((http://|ftp://)?([a-zA-Z1-9\-_\.]+\.[a-zA-Z1-9\-_]+\.[a-z]{2,3}(/[a-z\-/\._\?=&]+)?))", "<a href=\"\\1\\2\" target=\"_blank\">\\0</a>", $string);

to convert every url to an link.
but if you have an url like www.php.net, it will create a link like <a href="www.php.net">, wich will not work.
To prevent this, I use

$string = preg_replace("((href=\")([a-zA-Z1-9\-_\.]+\.[a-zA-Z1-9\-_]+\.[a-z]{2,3}(/[a-z\-/\._\?=&]+)?))", "href=\"http://\\2", $string);

wich will replace anything like href="something.php.net with href="http://something.php.net

I think it will be nice for other people to include (something like) this function in php, because many people would use it

thanks