Function issue with Walker_Nav_Menu [duplicate]

As you can see here, the declaration of this function looks like this:

public function start_lvl( &$output, $depth = 0, $args = array() ) {

So in your class it should look the same – it should have 3 params. But in your code it has only 2:

function start_lvl(&$output, $depth) {

Also… You shouldn’t change visibility of that function, so it also should be public.

So you should change your code so it looks like this:

...
public function start_lvl( &$output, $depth = 0, $args = array() ) {
    $indent = str_repeat("\t", $depth);
    $output .= "\n$indent<ul class=\"flyout\">\n";
}
...