您可以使用以下内容:

$string = "Look on http://www.google.com";
$string = preg_replace(
              "~[[:alpha:]]+://[^<>[:space:]]+[[:alnum:]/]~",
              "<a href="\0">\0</a>", 
              $string);

PHP 版本 <5.3 (ereg_replace) 否则 (preg_replace)