A formatter is an object that is responsible for taking an event array describing a log event and outputting a string with a formatted log line.

Some writers are not line-oriented, such as the Db, FirePhp, and ChromePhp writers. For these writers, formatters ensure the individual values in the event array are formatted properly for the writer.

Simple Formatting

Zend\Log\Formatter\Simple is the default formatter. It is configured automatically when you specify no formatter. The default configuration is equivalent to the following:

$format = '%timestamp% %priorityName% (%priority%): %message%' . PHP_EOL;
$formatter = new Zend\Log\Formatter\Simple($format);

A formatter is set on an individual writer object using the writer's setFormatter() method:

$writer = new Zend\Log\Writer\Stream('php://output');
$formatter = new Zend\Log\Formatter\Simple('hello %message%' . PHP_EOL);

$logger = new Zend\Log\Logger();


// outputs "hello there"

The constructor of Zend\Log\Formatter\Simple accepts a single parameter: the format string. This string contains keys surrounded by percent signs (e.g. %message%). The format string may contain any key from the event data array. You can retrieve the default keys by using the DEFAULT_FORMAT constant from Zend\Log\Formatter\Simple.

Formatting to XML

Zend\Log\Formatter\Xml formats log data into XML strings. By default, it automatically logs all items in the event array:

$writer = new Zend\Log\Writer\Stream('php://output');
$formatter = new Zend\Log\Formatter\Xml();

$logger = new Zend\Log\Logger();

$logger->info('informational message');

The code above outputs the following XML (space added for clarity):

  <message>informational message</message>

It's possible to customize the root element, as well as specify a mapping of XML elements to the items in the event data array. The constructor of Zend\Log\Formatter\Xml accepts a string with the name of the root element as the first parameter, and an associative array with the element mapping as the second parameter:

$writer = new Zend\Log\Writer\Stream('php://output');
$formatter = new Zend\Log\Formatter\Xml('log', [
    'msg' => 'message',
    'level' => 'priorityName',

$logger = new Zend\Log\Logger();

$logger->info('informational message');

The code above changes the root element from its default of logEntry to log. It also maps the element msg to the event data item message. This results in the following output:

  <msg>informational message</msg>

Formatting to FirePhp

Zend\Log\Formatter\FirePhp formats log data for the Firebug extension for Firefox.