How to add anchor tag to a URL from text input

First, a request. Don’t do this before writing the data to the database. Instead, do it before displaying the data to the end-user. This will cut down on all confusion, and will allow you more flexibility in the future.

One example found online follows:

$text = preg_replace('@(https?://([-\w\.]+)+(:\d+)?(/([-\w/_\.]*(\?\S+)?)?)?)@', '<a href="https://stackoverflow.com/questions/1959062/">$1</a>', $text);

And a much more thorough one from daringfireball.net:

/**
 * Replace links in text with html links
 *
 * @param  string $text
 * @return string
 */
function auto_link_text($text)
{
   $pattern  = '#\b(([\w-]+://?|www[.])[^\s()<>]+(?:\([\w\d]+\)|([^[:punct:]\s]|/)))#';
   $callback = create_function('$matches', '
       $url       = array_shift($matches);
       $url_parts = parse_url($url);

       $text = parse_url($url, PHP_URL_HOST) . parse_url($url, PHP_URL_PATH);
       $text = preg_replace("/^www./", "", $text);

       $last = -(strlen(strrchr($text, "https://stackoverflow.com/"))) + 1;
       if ($last < 0) {
           $text = substr($text, 0, $last) . "&hellip;";
       }

       return sprintf(\'<a rel="nowfollow" href="https://stackoverflow.com/questions/1959062/%s">%s</a>\', $url, $text);
   ');

   return preg_replace_callback($pattern, $callback, $text);
}