Logging in Drupal 7

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