How do I linkify urls in a string with php?

lib_autolink does a pretty good job, avoiding pitfalls like extra punctuation after the link and links inside HTML tags:

https://github.com/iamcal/lib_autolink

You can use the following:

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

PHP versions < 5.3 (ereg_replace) otherwise (preg_replace)

Have a look at regular expressions. You would then do something like:

$text = preg_replace('@(https?://([-\w\.]+)+(:\d+)?(/([\w/_\.]*(\?\S+)?)?)?)@', '<a href="$1">$1</a>', $text);