Method 'x' of class 'y' is not declared STATIC

Sample code:

class y {
    public function x($number) {
        //...
    }
    //...
}
//...
y::x(12);                       // Method 'x' of class 'y' is not declared STATIC
//...

You tried to call method of a class as it was static although it was not. In PHP this is considered legal as long as you are not using $this inside function. However Codenizer will report this as error.