There are two ways to reference the
$wpdb object. The first is to use the PHP keyword
global in order to act on the global instance of the object.
global $wpdb; echo $wpdb->prefix; // Outputs the prefix for the database
The second way to use the
$wpdb object is to reference PHP's
$GLOBALS super global variable.
echo $GLOBALS['wpdb']->prefix; // This will also output the prefix for the database
The second way is discouraged as it may not be considered the best practice.