This handler what is not yet part of standard Monolog distribution, is available on Packagist bartlett/monolog-callbackfilterhandler and as such installable via Composer.

$ php composer.phar require bartlett/monolog-callbackfilterhandler

We will create an anonymous function to filter only record on fake error messages with exception as contextual data.

CallbackFilterHandler class constructor accept an array of callback functions. Here is an example of the definition corresponding to our goal.

<?php
$filters = array(
    function ($record) {
        if (!array_key_exists('exception', $record['context'])) {
            return false;
        }
        return (preg_match('/fake error/', $record['message']) === 1);
    }
);

This script used the first filter described previously, on a important notification demo system, while all events are logged to a simple file.

<?php

use Bartlett\Monolog\Handler\CallbackFilterHandler;

use Monolog\Logger;
use Monolog\Handler\RotatingFileHandler;
use Monolog\Handler\StreamHandler;

// Create the logger
$logger = new Logger('my_logger');

// Create filter rules
$filters = array(
    function ($record) {
        if (!array_key_exists('exception', $record['context'])) {
            return false;
        }
        return (preg_match('/fake error/', $record['message']) === 1);
    }
);

// Create some handlers
$stream = new RotatingFileHandler(__DIR__ . DIRECTORY_SEPARATOR . 'my_logger.log');
$stream->setFilenameFormat('{filename}-{date}', 'Ymd');

$mailer = new StreamHandler(__DIR__ . DIRECTORY_SEPARATOR . 'notifications.log', Logger::ERROR);

// add handlers to the logger
$logger->pushHandler($stream);
$logger->pushHandler(new CallbackFilterHandler($mailer, $filters));

// You can now use your logger
$logger->addInfo('My logger is now ready');

$logger->addError('A fake error has occured. Will be logged to file BUT NOT notified by mail.');

try {
    throw new \RuntimeException();

} catch (\Exception $e) {
    $logger->addCritical(
        'A fake error has occured. Will be logged to file AND notified by mail.',
        array('exception' => (string) $e)
    );
}

Let’s review what we’ve done :