Подробные видеоинструкции Wordpress на тему: "Non-static method should not be called statically in wordpress":
Wordpress: PHP Deprecated: Non-static method should not be called statically (2 Solutions!!)
Deprecated Non static method HTML::xyz should not be called statically: (Fixed)
7.14 Why Non-Static variables does not work in Static method in Java?
why non-static method or field can't be called from a static method?
static methods, fix non static variable cannot be referenced