In case you need to log errors and other messages in Drupal 7 use may use it’s watchdog function. Same thing will work for Drupal 6.
watchdog($type, $message, $variables = array(), $severity = WATCHDOG_NOTICE, $link = NULL)
Example with some data:
watchdog('Booking', 'Sucessfully booked for %persons', array('%persons' => 5), WATCHDOG_INFO);
Some possible severity levels are WATCHDOG_EMERGENCY, WATCHDOG_ALERT, WATCHDOG_CRITICAL, WATCHDOG_ERROR, WATCHDOG_WARNING, WATCHDOG_NOTICE, WATCHDOG_INFO, WATCHDOG_DEBUG.
Read more from Drupal 7 documentation