yii\helpers\BaseInflector::underscore PHP Method

Converts any "CamelCased" into an "underscored_word".

public static underscore ( string $words ) : string
$words	string	the word(s) to underscore
return	string

    public static function underscore($words)
    {
        return strtolower(preg_replace('/(?<=\\w)([A-Z])/', '_\\1', $words));
    }

BaseInflector