The problem here is caused by the fact that you are in namespace Blog\View\Helper
as declared in your code, and since you call the class tidy
without its fully qualified name, PHP supposes you actually want Blog\View\Helper\tidy
.
Simply do $tidy = new \tidy();
and you should be fine, notice the prepended slash.
You might also want to read more about namespaces in PHP: http://php.net/manual/en/language.namespaces.php
Also, you might need to do use the fully qualified class name for all the other classes from the root namespace, and in general from namespaces other than the current one, or issue the proper use
statement, as explained here: http://php.net/manual/en/language.namespaces.importing.php