framework/BaseYii.php
public static function debug($message, $category = 'application')
    {
        if (YII_DEBUG) {
            static::getLogger()->log($message, Logger::LEVEL_TRACE, $category);
        }
    }
public static function debug($message, $category = 'application')
    {
        if (YII_DEBUG) {
            static::getLogger()->log($message, Logger::LEVEL_TRACE, $category);
        }
    }
Logs a debug message.
Trace messages are logged mainly for development purpose to see
the execution work flow of some code. This method will only log
a message when the application is in debug mode.
@param string|array $message the message to be logged. This can be a simple string or a more
complex data structure, such as array.
@param string $category the category of the message.
@since 2.0.14