¿como convertir un texto en una url?

[fran86]

Gryphus, la funcion es la solucion a lo que requieres, pero la paso de nuevo (con los creditos del autor), porque ahora que me doy cuenta, usando el codigo vB de [ PHP ], imprime cualquier cosa y daba un error "Compilation failed: range out of order in character class at offset..."

/**
* Rewritten by Nathan Codding - Feb 6, 2001.
* - Goes through the given string, and replaces xxxx://yyyy with an HTML <a> tag linking
* to that URL
* - Goes through the given string, and replaces www.xxxx.yyyy[zzzz] with an HTML <a> tag linking
* to http://www.xxxx.yyyy[/zzzz]
* - Goes through the given string, and replaces xxxx@yyyy with an HTML mailto: tag linking
* to that email address
* - Only matches these 2 patterns either after a space, or at the beginning of a line
*
* Notes: the email one might get annoying - it's easy to make it more restrictive, though.. maybe
* have it require something like [email protected] or such. We'll see.
*/
function make_clickable($text)
{

// pad it with a space so we can match things at the start of the 1st line.
$ret = ' ' . $text;

// matches an "xxxx://yyyy" URL at the start of a line, or after a space.
// xxxx can only be alpha characters.
// yyyy is anything up to the first space, newline, comma, double quote or <
$ret = preg_replace("#([\t\r\n ])([a-z0-9]+?){1}://([\w\-]+\.([\w\-]+\.)*[\w]+(:[0-9]+)?(/[^ \"\n\r\t<]*)?)#i", '\1<a href="\2://\3" target="_blank">\2://\3</a>', $ret);

// matches a "www|ftp.xxxx.yyyy[/zzzz]" kinda lazy URL thing
// Must contain at least 2 dots. xxxx contains either alphanum, or "-"
// zzzz is optional.. will contain everything up to the first space, newline,
// comma, double quote or <.
$ret = preg_replace("#([\t\r\n ])(www|ftp)\.(([\w\-]+\.)*[\w]+(:[0-9]+)?(/[^ \"\n\r\t<]*)?)#i", '\1<a href="http://\2.\3" target="_blank">\2.\3</a>', $ret);

// matches an email@domain type address at the start of a line, or after a space.
// Note: Only the followed chars are valid; alphanums, "-", "_" and or ".".
$ret = preg_replace("#([\n ])([a-z0-9\-_.]+?)@([\w\-]+\.([\w\-\.]+\.)*[\w]+)#i", "\\1<a href=\"mailto:\\2@\\3\">\\2@\\3</a>", $ret);

// Remove our padding..
$ret = substr($ret, 1);

return($ret);
}