How to use $modx->log()

$modx->setLogLevel(xPDO::LOG_LEVEL_INFO);

$target = array();
$target['target'] = 'FILE';
$target['options']['filename'] = 'test.log';

$modx->log( modX::LOG_LEVEL_ERROR, 'xyzzy', $target);

<?php
# $modx->setLogLevel(xPDO::LOG_LEVEL_INFO);

$modx->setLogTarget(array(  
	'target' => 'FILE',  
	'options' => array(  
		'filename' => 'test.log'
	)  
));

$modx->log( modX::LOG_LEVEL_ERROR, 'xyzzy');

<?php
# $modx->setLogLevel(xPDO::LOG_LEVEL_INFO);

$target = array(  
	'target' => 'FILE',  
	'options' => array(  
		'filename' => 'test.log'
	)  
);
$modx->setLogTarget($target);

$modx->log( modX::LOG_LEVEL_ERROR, 'xyzzy', $target);

public function setLogTarget($target= 'ECHO') {
        $oldTarget = $this->logTarget;
        $this->logTarget= $target;
        return $oldTarget;
    }

 /**
     * Log a message with details about where and when an event occurs.
     *
     * @param integer $level The level of the logged message.
     * @param string $msg The message to log.
     * @param string $target The logging target.
     * @param string $def The name of a defining structure (such as a class) to
     * help identify the message source.
     * @param string $file A filename in which the log event occured.
     * @param string $line A line number to help locate the source of the event
     * within the indicated file.
     */
    public function log($level, $msg, $target= '', $def= '', $file= '', $line= '') {
        $this->_log($level, $msg, $target, $def, $file, $line);
    }

<?php
$modx->setLogTarget('FILE');

$target = array(
    'target' => 'FILE',
    'options' => array(
        'filename' => 'path_to_file'),
);
$modx->log(xPDO::LOG_LEVEL_ERROR, 'Error Message', $target);