The function output_form in categories.class.php expects an array in $form_input. When the function get called to create the form for a new category, the function call provided an emptry string. This led to a warning since PHP 5.4 because of type mismatch. Providing an array prevents this error. This is the only pace, I found the error.
Mostly changing mysql_ to mysqli_ but also some OOP fixes it classes,
especially the bbcode parser which doesn't have a new version working
with PHP 7.
