Processors allow you to provide additional information to logs in an automated fashion. They are called from the logger before the event is passed to the writers; they receive the event array, and return an event array on completion.

Use cases include:

  • Providing exception backtrace information.
  • Injecting substitutions into the message.
  • Injecting a request identifier (in order to later inspect logs for a specific identifier)

Processor interface

All processors must implement Zend\Log\Processor\ProcessorInterface:

namespace Zend\Log\Processor;

interface ProcessorInterface
     * Processes a log message before it is given to the writers
     * @param  array $event
     * @return array
    public function process(array $event);

Adding processors

To add a processor to a Logger instance, inject it using the addProcessor() method:

$logger->addProcessor(new Zend\Log\Processor\Backtrace());

Available processors

The following processors are available.


Zend\Log\Processor\Backtrace calls debug_backtrace() for every log event, injecting the details into the event's extra array:

$event = [
    // ... standard elements ...
    'extra' => [
        'file' => 'SomeFile.php',
        'line' => 1337,
        'class' => 'Foo\MyClass',
        'function' => 'myMethod',


Zend\Log\Processor\PsrPlaceholder replaces PSR-3-formatted message placeholders with the values found in the extra array.

As an example:

$logger->addProcessor(new Zend\Log\Processor\PsrPlaceholder());
$logger->warn('Invalid plugin {plugin}', ['plugin' => 'My\Plugins\FooPlugin']);

will output:

Invalid plugin My\Plugins\FooPlugin

This feature allows compatibility with PSR-3, and provides a simple way to provide string substitutions without needing to resort to sprintf() in your userland code.


Zend\Log\Processor\ReferenceId allows you to specify a static reference identifier to inject in all log messages; typically, you will generate a new one for each request, to allow querying logs for the given reference identifier later.

Given the following:

$processor = new Zend\Log\Processor\ReferenceId();
$processor->setIdentifier(microtime(true) . '_' . uniqid());
$logger->info('Log event');

The event will contain:

$event = [
    /* ... standard values ... */
    'extra' => [
        'referenceId' => '1455057110.6284_56ba68ebe1244',


Zend\Log\Processor\RequestId is similar to ReferenceId with one key difference: if you do not set an identifier, one is automatically generated for you using hashed information from $_SERVER, including REQUEST_TIME_FLOAT, HTTP_X_FORWARDED_FOR, and/or REMOTE_ADDR.

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