This is a simple MySQLi extender example. You instantiate this class just as you would MySQLi.

<?php
include_once('mysqli.extend.class.php');
$mysql = new mysqli_db('dbhost', 'dbuser', 'dbpass', 'dbname');
?>

The multi_fetch_array() function essentially allows you to preform multiple queries. This is particularly handy when needing to call multiple stored procedures concurrently.
Example:

<?php
$m[] = sprintf('CALL fetchUserInfo(\'%s\')', $username);
$m[] = sprintf('CALL fetchUserActivity(\'%s\')', $username);
$result = $mysql->multi_fetch_array( implode(';', $m) );
?>

<?php
class mysqli_db extends mysqli {
  public function fetch_array($sql) {
    $query = parent::query($sql);
    if ($query) {
      while( $result = $query->fetch_assoc() ) $return[] = $result;
      $query->free();
    }
    return ( (isset($return)) ? $return : false );
  } 

  public function multi_fetch_array($sql) {
    if (parent::multi_query($sql)) {
      do {
        if ($result = parent::store_result()) {
          while ($row = $result->fetch_assoc()) $return[] = $row;
          $result->free();
        }
      } while (parent::next_result());
    }
    return $return;
  }
}
?>