A Logger is created by calling the Zend_Log::registerLogger() method with a Zend_Log_Adapter_Interface instance as an argument.

require_once 'Zend/Log.php';                // Zend_Log base class
require_once 'Zend/Log/Adapter/File.php';   // File log adapter

// Register the file logger
Zend_Log::registerLogger(new Zend_Log_Adapter_File('/logs/simple.txt'));

Messages are then sent to the log with Zend_Log::log().

/**
 * Send a message to the log.  An optional second argument specifies the log
 * level.  If no level is supplied, LEVEL_DEBUG is assumed.
 */
Zend_Log::log('First logging message.');

/**
 * Log a severe message.
 */
Zend_Log::log('A serious error has occurred.', Zend_Log::LEVEL_SEVERE);