ModuleHandler::alter

public ModuleHandler::alter($type, &$data, &$context1 = NULL, &$context2 = NULL)

  $context = array(
    'alterable' => &$alterable,
    'unalterable' => $unalterable,
    'foo' => 'bar',
  );
  $this->alter('mymodule_data', $alterable1, $alterable2, $context);

  $context = array(
    'unalterable_object' => clone $object,
  );
  $this->alter('mymodule_data', $data, $context);

public function alter($type, &$data, &$context1 = NULL, &$context2 = NULL) {
  // Most of the time, $type is passed as a string, so for performance,
  // normalize it to that. When passed as an array, usually the first item in
  // the array is a generic type, and additional items in the array are more
  // specific variants of it, as in the case of array('form', 'form_FORM_ID').
  if (is_array($type)) {
    $cid = implode(',', $type);
    $extra_types = $type;
    $type = array_shift($extra_types);
    // Allow if statements in this function to use the faster isset() rather
    // than !empty() both when $type is passed as a string, or as an array
    // with one item.
    if (empty($extra_types)) {
      unset($extra_types);
    }
  }
  else {
    $cid = $type;
  }

  // Some alter hooks are invoked many times per page request, so store the
  // list of functions to call, and on subsequent calls, iterate through them
  // quickly.
  if (!isset($this->alterFunctions[$cid])) {
    $this->alterFunctions[$cid] = array();
    $hook = $type . '_alter';
    $modules = $this->getImplementations($hook);
    if (!isset($extra_types)) {
      // For the more common case of a single hook, we do not need to call
      // function_exists(), since $this->getImplementations() returns only
      // modules with implementations.
      foreach ($modules as $module) {
        $this->alterFunctions[$cid][] = $module . '_' . $hook;
      }
    }
    else {
      // For multiple hooks, we need $modules to contain every module that
      // implements at least one of them.
      $extra_modules = array();
      foreach ($extra_types as $extra_type) {
        $extra_modules = array_merge($extra_modules, $this->getImplementations($extra_type . '_alter'));
      }
      // If any modules implement one of the extra hooks that do not implement
      // the primary hook, we need to add them to the $modules array in their
      // appropriate order. $this->getImplementations() can only return
      // ordered implementations of a single hook. To get the ordered
      // implementations of multiple hooks, we mimic the
      // $this->getImplementations() logic of first ordering by
      // $this->getModuleList(), and then calling
      // $this->alter('module_implements').
      if (array_diff($extra_modules, $modules)) {
        // Merge the arrays and order by getModuleList().
        $modules = array_intersect(array_keys($this->moduleList), array_merge($modules, $extra_modules));
        // Since $this->getImplementations() already took care of loading the
        // necessary include files, we can safely pass FALSE for the array
        // values.
        $implementations = array_fill_keys($modules, FALSE);
        // Let modules adjust the order solely based on the primary hook. This
        // ensures the same module order regardless of whether this if block
        // runs. Calling $this->alter() recursively in this way does not
        // result in an infinite loop, because this call is for a single
        // $type, so we won't end up in this code block again.
        $this->alter('module_implements', $implementations, $hook);
        $modules = array_keys($implementations);
      }
      foreach ($modules as $module) {
        // Since $modules is a merged array, for any given module, we do not
        // know whether it has any particular implementation, so we need a
        // function_exists().
        $function = $module . '_' . $hook;
        if (function_exists($function)) {
          $this->alterFunctions[$cid][] = $function;
        }
        foreach ($extra_types as $extra_type) {
          $function = $module . '_' . $extra_type . '_alter';
          if (function_exists($function)) {
            $this->alterFunctions[$cid][] = $function;
          }
        }
      }
    }
  }

  foreach ($this->alterFunctions[$cid] as $function) {
    $function($data, $context1, $context2);
  }
}