- Code and documentation improvements by JonBob.

includes/module.inc

 <?php
 // $Id$
 
-// initialize modules:
+/**
+ * Initialize all modules.
+ *
+ * To change the required set of modules, change this function as well as
+ * system_listing() and module_list().
+ */
 function module_init() {
-  // Note: changing this also requires changing system_listing() @ modules/system.module.
-  require_once "modules/admin.module";
-  require_once "modules/filter.module";
-  require_once "modules/system.module";
-  require_once "modules/user.module";
-  require_once "modules/watchdog.module";
+  require_once 'modules/admin.module';
+  require_once 'modules/filter.module';
+  require_once 'modules/system.module';
+  require_once 'modules/user.module';
+  require_once 'modules/watchdog.module';
   module_list();
-  module_invoke_all("init");
+  module_invoke_all('init');
 }
 
-// apply function $function to every known module:
-function module_iterate($function, $argument = "") {
+/**
+ * Call a function repeatedly with each module in turn as an argument.
+ */
+function module_iterate($function, $argument = '') {
   foreach (module_list() as $name) {
     $function($name, $argument);
   }
 }
 
-// invoke hook $hook of module $name with optional arguments:
-function module_invoke($name, $hook, $a1 = NULL, $a2 = NULL, $a3 = NULL, $a4 = NULL) {
-  $function = $name ."_". $hook;
-  if (function_exists($function)) {
-    return $function($a1, $a2, $a3, $a4);
-  }
-}
-
-// invoke $hook for all appropriate modules:
-function module_invoke_all($hook, $a1 = NULL, $a2 = NULL, $a3 = NULL, $a4 = NULL) {
-  $return = array();
-  foreach (module_list() as $name) {
-    $result = module_invoke($name, $hook, $a1, $a2, $a3, $a4);
-    if (isset($result)) {
-      $return = array_merge($return, $result);
-    }
-  }
-
-  return $return;
-}
-
-// return array of module names (includes lazy module loading if not in bootstrap mode)
-function module_list($refresh = 0, $bootstrap = 0) {
+/**
+ * Collect a list of all installed and enabled modules.
+ *
+ * If any modules are enabled but have not yet been loaded, this function performs
+ * the include_once() to load them.
+ *
+ * @param $refresh
+ *   Whether to force the module list to be regenerated (such as after the
+ *   administrator has changed the system settings).
+ * @param $bootstrap
+ *   Whether to return the reduced set of modules loaded in "bootstrap mode" for
+ *   cached pages. See bootstrap.inc.
+ * @return
+ *   An associative array whose keys and values are the names of all loaded
+ *   modules.
+ */
+function module_list($refresh = FALSE, $bootstrap = FALSE) {
   static $list;
 
   if ($refresh) {
@@ -50,7 +50,7 @@ function module_list($refresh = 0, $bootstrap = 0) {
   }
 
   if (!$list) {
-    $list = array("admin" => "admin", "filter" => "filter", "system" => "system", "user" => "user", "watchdog" => "watchdog");
+    $list = array('admin' => 'admin', 'filter' => 'filter', 'system' => 'system', 'user' => 'user', 'watchdog' => 'watchdog');
     if ($bootstrap) {
       $result = db_query("SELECT name, filename, throttle, bootstrap FROM {system} WHERE type = 'module' AND status = 1 AND bootstrap = 1");
     }
@@ -59,12 +59,10 @@ function module_list($refresh = 0, $bootstrap = 0) {
     }
     while ($module = db_fetch_object($result)) {
       if (file_exists($module->filename)) {
-        /*
-        ** Determine the current throttle status and see if module should be
-        **  loaded based on server load.  We have to directly access the
-        **  throttle variables as the throttle.module may not be loaded yet.
-        */
-        $throttle = ($module->throttle && variable_get("throttle_level", 0) > 4);
+        // Determine the current throttle status and see if the module should be
+        // loaded based on server load. We have to directly access the throttle
+        // variables, since throttle.module may not be loaded yet.
+        $throttle = ($module->throttle && variable_get('throttle_level', 0) > 4);
         if (!$throttle) {
           $list[$module->name] = $module->name;
           include_once $module->filename;
@@ -76,15 +74,99 @@ function module_list($refresh = 0, $bootstrap = 0) {
   return $list;
 }
 
-// return 1 if module $name exists, 0 otherwise:
-function module_exist($name) {
+/**
+ * Determine whether a given module exists.
+ *
+ * @param $module
+ *   The name of the module (without the .module extension).
+ * @return
+ *   TRUE if the module is both installed and enabled.
+ */
+function module_exist($module) {
   $list = module_list();
-  return isset($list[$name]) ? 1 : 0;
+  return array_key_exists($module, $list);
 }
 
-// return 1 if module $name implements hook $hook, 0 otherwise:
-function module_hook($name, $hook) {
-  return function_exists($name ."_". $hook);
+/**
+ * @defgroup hooks Hooks
+ *
+ * Drupal's module system is based on the concept of "hooks". A hook is a PHP
+ * function that is named foo_bar(), where "foo" is the name of the module (whose
+ * filename is thus foo.module) and "bar" is the name of the hook. Each hook has
+ * a defined set of parameters and a specified result type.
+ *
+ * To extend Drupal, a module need simply implement a hook. When Drupal wishes to
+ * allow intervention from modules, it determines which modules implement a hook
+ * and call that hook in all enabled modules that implement it.
+ *
+ * The available hooks to implement are explained here in the Hooks section of
+ * the developer documentation. The string "hook" is used as a placeholder for
+ * the module name is the hook definitions. For example, if the module file is
+ * called example.module, then hook_help() as implemented by that module would be
+ * defined as example_help().
+ *
+ * @{
+ */
+
+/**
+ * Determine whether a module implements a hook.
+ *
+ * @param $module
+ *   The name of the module (without the .module extension).
+ * @param $hook
+ *   The name of the hook (e.g. "help" or "menu").
+ * @return
+ *   TRUE if the module is both installed and enabled, and the hook is
+ *   implemented in that module.
+ */
+function module_hook($module, $hook) {
+  return function_exists($module .'_'. $hook);
 }
 
+/**
+ * Invoke a hook in a particular module.
+ *
+ * @param $module
+ *   The name of the module (without the .module extension).
+ * @param $hook
+ *   The name of the hook to invoke.
+ * @param ...
+ *   Arguments to pass to the hook implementation.
+ * @return
+ *   The return value of the hook implementation.
+ */
+function module_invoke($module, $hook, $a1 = NULL, $a2 = NULL, $a3 = NULL, $a4 = NULL) {
+  $function = $module .'_'. $hook;
+  if (function_exists($function)) {
+    return $function($a1, $a2, $a3, $a4);
+  }
+}
+
+/**
+ * Invoke a hook in all enabled modules that implement it.
+ *
+ * @param $hook
+ *   The name of the hook to invoke.
+ * @param ...
+ *   Arguments to pass to the hook.
+ * @return
+ *   An array of return values of the hook implementations. If modules return
+ *   arrays from their implementations, those are merged into one array.
+ */
+function module_invoke_all($hook, $a1 = NULL, $a2 = NULL, $a3 = NULL, $a4 = NULL) {
+  $return = array();
+  foreach (module_list() as $module) {
+    $result = module_invoke($module, $hook, $a1, $a2, $a3, $a4);
+    if (isset($result)) {
+      $return = array_merge($return, $result);
+    }
+  }
+
+  return $return;
+}
+
+/**
+ * @} end of defgroup hooks
+ */
+
 ?>