What is the "Drupal" Way of using Session Variables?

The way I understand it, $_SESSION values are session-specific since they last only as long as the user's session. variable_set() is used for module and site configuration variables. If the variables you want to use in your blocks are intended to last only for the user's current session, then use $_SESSION['var']. If you want values to persist across sessions, you'll probably need to create a module that uses its own table to store user variables.

I wrote some code which I've been considering packaging as a module... enclosed herein. This code gives anonymous users the benefit of "remember me" and registered users "remember me, indefinitely, from any computer." You can see it in action at www.authorcollector.com.

- If a user is registered (has a non-zero UID), preferences are stored in a database (in the code below, database 'user_preferences').
- Preferences for unregistered users are stored in a session variable $_SESSION['my_module'].
- Data is serialized into a single value field, so no prior knowledge of field names or structure is required.
- If a value is retrieved which has not been set, you'll get an empty string. I suppose this could be improved to support defaults or changed to return a NULL.
- Note this code makes no attempt to sanity check the input (no check_plain)... this is left up to the client.

Usage:
1) Getting a single session variable...

$prefs = get_user_preferences('customer_name');
// $prefs contains the session value for 'customer_name' (or is an empty string is never set)
// I find the single value syntax useful for forms... i.e. '#default_value' => get_user_preferences('customer_name').

2) Getting multiple session variables...

$prefs = get_user_preferences(array('customer_name', 'customer_address'));
// $prefs['customer_name'] and $prefs['customer_address'] are populated (or empty strings if they'd never been set)

3) Setting one or more session variables...

set_user_preferences(array('customer_name' => 'Mark Allen Neil'));
set_user_preferences(array('customer_name' => 'Mark Allen Neil', 'customer_address' => '123 Main Street'));

Here is the code... I generally package this as 'user.prefs.inc' and then include it in files which manage preferences.

<?php
// $Id: user.prefs.inc $

/**
 * @file
 * Saves and loads user preferences.
 * 
 * Registered users preferences are stored in a database, anonymous user data in session.
 *
 */
 
function get_user_preferences($input) { 
  $prefs = _get_user_preferences(); 
  if (is_array($input)) {    // Input is an array of names
    $output = array();
    foreach ($input as $name) {
      if (isset($prefs[$name])) {    
        $output[$name] = $prefs[$name];
      }
      else {
        $output[$name] = '';
      }
    }
  }
  else {                    // Input is a single name
    if (isset($prefs[$input])) {    
      $output = $prefs[$input];
    }
    else {
      $output = '';
    }
  }
  return $output;
}

function set_user_preferences($in_prefs) {
  $prefs = _get_user_preferences();
  foreach ($in_prefs as $name => $value) {
    $prefs[$name] = $value;
  } 
  _set_user_preferences($prefs);
}

function _get_user_preferences() {
  global $user;
  if ($user->uid) { // Registered user, preferences in database
    $result = db_query("SELECT value FROM {user_preferences} WHERE uid = '%d'", $user->uid);
    if ($result) {
      return unserialize(db_result($result));
    }   
  }
  else {            // Anonymous user, preferences in cookies
    if (isset($_SESSION['my_module'])) {
      return $_SESSION['my_module'];      
    } 
  }
  return array();  
}

function _set_user_preferences($prefs) {
  global $user;
  if ($user->uid) { // Registered user, preferences in database  
    $serialized_prefs = serialize($prefs); 
    db_query("UPDATE {user_preferences} SET value = '%s' WHERE uid = '%d'", $serialized_prefs, $user->uid);
    if (!db_affected_rows()) {
      @db_query("INSERT INTO {user_preferences} (uid, value) VALUES ('%d', '%s')", $user->uid, $serialized_prefs);
    }
  }
  else {            // Anonymous user, preferences in session
    if (!isset($_SESSION['my_module'])) {
      $_SESSION['my_module'] = array();
    }
    foreach ($prefs as $name => $value) {
      $_SESSION['my_module'][$name] = $value;
    }
  }
}

And the following is the code in your module's install file... "my_module.install"...

function my_module_schema() {
  $schema['user_preferences'] = array(
    'fields' => array(   
      'uid' => array('type' => 'int', 'not null' => 'TRUE', 'default' => '0'),
      'value' => array('type' => 'text', 'size' => 'big', 'not null' => TRUE),     
    ),
    'primary key' => array('uid'),
  );   
  return $schema;
}