When I'm coding modules, I rely heavily on the information found here: http://api.drupal.org/api/functions. Searching for functions can show you what already exists and how to use them in your code. Before you code something from scratch (or start to write your own SQL statement), you should start with the API. To be honest, I can't say that I've never coding something from scratch.
Another way to figure out how Drupal functionality is generated is to check out the menu hook in the module. A lot of helpful information resides here. For instance, you might want to embed a forum in a block or a custom module. If the functionality you're trying to recreate is found on your site at http://YOURSITE/forum, look for the menu_hook entry for "forum". Open /modules/forum/forum.module, locate the forum_menu() function, and look in the associative array for the "forum" entry:
// it starts with:
$items['forum'] = array(In this case, the "page callback" property of the array tells you which callback function is being used. If you search the forum.module file, you notice that this function does not exist. The "file" property of the array tells you that this function requires an additional file to function properly, in this case "forum.pages.inc". At this point, it's helpful to review the code in the page callback function and use the online API documentation as a reference. NOTE: more information about hook_menu() can be found here: http://api.drupal.org/api/function/hook_menu/6.
For this example, I decided to use the forum_get_forum() function in my code. Before I can simply call this function, I must include the same file defined in the "file" property of the menu_hook entry:
<?php
require_once(drupal_get_path('module','forum') . '/forum.pages.inc');
$forums = forum_get_forums(0);
?>Now, I have the html generated from the forum module. You could use this same process to recreate functionality from other modules as well.
