$msg = ‘No content from %url.’;
$vars = array( ‘%url’ => $url );
watchdog(‘module_name’, $msg, $vars, WATCHDOG_WARNING);

There are eight different constants that can be passed to this function:
WATCHDOG_EMERG: The system is now in an unusable state.
WATCHDOG_ALERT: Something must be done immediately.
WATCHDOG_CRITICAL: The application is in a critical state.
WATCHDOG_ERROR: An error occurred.
WATCHDOG_WARNING: Something unexpected (and negative) happened, but
didn’t cause any serious problems.
WATCHDOG_NOTICE: Something significant (but not bad) happened.
WATCHDOG_INFO: Information can be logged.
WATCHDOG_DEBUG: Debugging information can be logged.